Add a provisional section on pytest code style w.r.t. fixtures (#2220)

### Changes
As stated in the title

### Reason for changes
This doesn't seem obvious to some developers, so will state this in the style guide.

### Related tickets
N/A

### Tests
N/A

docs/styleguide/PyGuide.md

@@ -31,6 +31,12 @@
       - [4.7.4 File Naming](#s4.7.4-file-naming)
     - [4.8 Main](#s4.8-main)
 - [5 API documentation rules](#s5-api-doc-rules)
+- [6 Test suite coding rules](#s6-test-suite-coding-rules)
+  - [6.1 Basic style](#61-basic-style)
+  - [6.2 Parametrization](#62-parametrization)
+  - [6.3 Folder structure](#63-folder-structure)
+  - [6.4 Test runtime considerations](#64-test-runtime-considerations)
+  - [6.5 BKC management](#65-bkc-management)
 
 </details>
 
@@ -226,6 +232,9 @@ with open("hello.txt") as hello_file:
         print(line)
 ```
 
+Use `pathlib.Path` instead of `os.*` methods for handling paths.
+It is preferrable to have `pathlib.Path` objects instead of `str` to represent file paths in the code logic if performance is not critical, converting these to `str` for external APIs that cannot handle `Path` objects.
+
 <a id="s3.7-abstract-classes"></a>
 <a id="37-abstract-classes"></a>
 <a id="abstract-classes"></a>
@@ -794,3 +803,132 @@ This is required so that the autogenerated API documentation is rendered properl
 
 If the autogenerated API documentation does not show type hints for certain arguments despite the fact that the type hints are present in the object's implementation code,
 or if the type hints do not refer to the API symbol's canonical alias, then the type hint should be explicitly declared in the docstring using the `:type *param_name*:` directive (or `:rtype:` for return types).
+
+<a id="s6-test-suite-coding-rules"></a>
+<a id="6-test-suite-coding-rules"></a>
+<a id="test-suite-coding-rules"></a>
+
+## 6 Test suite coding rules
+
+Unit tests are written and executed using the [pytest](https://pytest.org) framework.
+
+Do not test the functionality of third-party libraries.
+
+<a id="s61-basic-style"></a>
+<a id="61-basic-style"></a>
+<a id="basic-style"></a>
+
+### 6.1 Basic style
+
+Parameters of test cases and fixtures should be type-hinted just like any other function.
+Test cases should ideally all have docstrings, except for the simple ones which are sufficiently described either by their names or by the simplicity of the code.
+
+For purposes of setting up test environment for a given case, or reusing setup/teardown code, or parametrizing complex cases the [fixture](https://docs.pytest.org/en/7.4.x/explanation/fixtures.html#about-fixtures) approach should be used.
+[Fixture scoping](https://docs.pytest.org/en/7.4.x/how-to/fixtures.html#scope-sharing-fixtures-across-classes-modules-packages-or-session) should be leveraged depending on the use case - for instance, a class-scoped fixture can be utilized to provide a storage object for data gathered in individual cases inside the class, and to post-process this data after the test cases in the class have all been executed.
+The fixtures that are meant to be reused in multiple test files should be defined in a corresponding `conftest.py` file, instead of importing these where they are needed from another non-`conftest.py` file where they were defined - otherwise the linters will try to remove it as an unused import and also the import itself may have an effect on how the fixture is executed based on its scope.
+
+The xunit-style setup and teardown functions ([described here](https://docs.pytest.org/en/7.4.x/how-to/xunit_setup.html#xunitsetup)) are disallowed.
+Xunit-style testing seems to be outdated ("old", as stated in pytest docs) since, as it seems, everything that is accomplished using xunit-style methods can be done via fixtures with [associated benefits](https://docs.pytest.org/en/7.4.x/explanation/fixtures.html#improvements-over-xunit-style-setup-teardown-functions).
+
+<a id="s62-parametrization"></a>
+<a id="62-parametrization"></a>
+<a id="parametrization"></a>
+
+### 6.2 Parametrization
+
+Test cases may be parametrized either using a `@pytest.mark.parametrize` decorator (for simple parametrizations), or by using [parametrized fixtures](https://docs.pytest.org/en/7.4.x/how-to/fixtures.html#parametrizing-fixtures) in case the test requires complex parametrization, or in case the parameters have to be preprocessed in a complex fashion before being passed into the test.
+The parameter set ("test structs") for each test case should be represented as a collection of type-hinted class objects (or dataclass objects).
+Do not use tuples, or namedtuples, or dicts to represent an individual parameter - these cannot be typed or refactored effectively, and for complex cases it is hard to visually tell by looking at the definiton of the parameter (test struct) just what each sub-parameter means.
+A set of IDs should be defined manually or by using an `idfn` for each case defined by the structs so that it is easier to distinguish visually between the test cases.
+When instantiating a test struct object, specify init arguments explicitly as keywords for increased visibility.
+
+Bad:
+
+```python3
+@pytest.mark.parametrize("param", [(42, "foo", None), (1337, "bar", RuntimeError)])
+def test_case_parametrized(param):
+    assert function_call() == param[0]
+    assert another_object.attribute != param[1]
+    if param[2] is not None:
+        with pytest.raises(param[2]):
+            should_raise()
+```
+
+Good:
+
+```python3
+@dataclass
+class StructForTest:
+    expected_return_value: int
+    prohibited_attribute_value: str
+    raises: Optional[Type[Exception]]
+
+
+def idfn(x: StructForTest) -> str:
+    return f"{x.expected_return_value}-{x.prohibited_attribute_value}-{x.raises}"
+
+
+@pytest.mark.parametrize(
+    "ts",
+    [
+        StructForTest(
+            expected_return_value=42, prohibited_attribute_value="foo", raises=None
+        ),
+        StructForTest(
+            expected_return_value=1337,
+            prohibited_attribute_value="bar",
+            raises=RuntimeError,
+        ),
+    ],
+    idfn=idfn,
+)
+def test_case_parametrized(ts: StructForTest):
+    assert function_call() == ts.expected_return_value
+    assert another_object.attribute != ts.prohibited_attribute_value
+    if ts.raises is not None:
+        with pytest.raises(ts.raises):
+            should_raise()
+```
+
+<a id="s63-folder-structure"></a>
+<a id="63-folder-structure"></a>
+<a id="folder-structure"></a>
+
+### 6.3 Folder structure
+
+Test files should be grouped into directories based on what is being tested, and in the scope of which backend.
+Framework-specific tests go into framework-specific directories respectively.
+Tests that check only common code functionality without backend-specific dependencies go into the `common` folder.
+Test cases that reuse the same testing code for multiple frameworks or test templates should be located in the `cross_fw` folder.
+Auxiliary code (non-test-case) that could be reused across the tests should be stored in the `shared` folder.
+
+<a id="s64-test-runtime-considerations"></a>
+<a id="64-test-runtime-considerations"></a>
+<a id="test-runtime-considerations"></a>
+
+### 6.4 Test runtime considerations
+
+Test cases should strive to have a low runtime while still testing what they were meant to test.
+Use mocking approach (with `unittest.mock` or `pytest-mock` functionality) to simulate behaviour of heavy/compute-intensive functionality (especially external) to save runtime.
+If a test is templated to be run for many backends, and if the common tested code has a significant runtime, consider passing mocks into backend-specific interfaces instead of actually running the common code, and test common code part separately.
+
+<a id="s65-bkc-management"></a>
+<a id="65-bkc-management"></a>
+<a id="bkc-management"></a>
+
+### 6.5 BKC management
+
+Python-level requirements for test runs are specified in `requirements.txt` files at the top levels of the corresponding test subdirectories.
+The versions for each package requirement must be specified exactly.
+
+Bad (e.g. `tests/example/requirements.txt`):
+
+```bash
+torch
+```
+
+Good:
+
+```bash
+torch==2.1.0
+```