Fix use of FakeRepository in tests that change current dir

[richafrank]

outside of the repo root, e.g. via tmpdir_cwd,
or when running any test from a different working dir.

Uses the absolute path instead of relative.

I encountered this on a branch (not ready yet) where I was starting to use both fixtures in a single test:

test setup failed
@pytest.fixture
    def repository():
>       return FakeRepository()

.../pip-tools/tests/conftest.py:136: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

self = <tests.conftest.FakeRepository object at 0x103893460>

    def __init__(self):
>       with open("tests/test_data/fake-index.json") as f:
E       FileNotFoundError: [Errno 2] No such file or directory: 'tests/test_data/fake-index.json'

.../pip-tools/tests/conftest.py:41: FileNotFoundError

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• Provided the tests for the changes. (Existing tests still pass.)
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[AndydeCleyre]

Thanks, deterministic paths are definitely an improvement.