Metatest for swap mappers

[1ucian0]

Fixes #1461.

This PR is a second attempt to have a simple meta test for all the "swappers".

The test checks the output of the swapper to a ground truth DAG (one for each test/swapper) saved in as a pickle (in test/python/pickles/). If they need to be regenerated, the DAG candidate is compiled and run in a simulator and the count is checked before being saved.

How to add a swapper

Add the following class in test/python/transpiler/test_mappers.py

class TestsSomeSwap(CommonTestCases, QiskitTestCase):
    """ Test CommonTestCases using SomeSwap """
    pass_class = SomeSwap           # <- The pass class
    additional_args = {'seed': 42}  # <- In case SomeSwap.__init__ requieres additional arguments

How to add a test for all the swappers

Add the following method in the class CommonTestCases:

    def test_a_common_test(self):
        self.count = {'000': 512, '110': 512}  # <- The expected count for this circuit
        self.delta = 5      # <- This is delta for the AlmostEqual during the count check
        coupling_map = [[0, 1], [0, 2]]  # <- The coupling map for this specific test

        qr = QuantumRegister(3, 'q')                       #
        cr = ClassicalRegister(3, 'c')                     #  Set the circuit to test
        circuit = QuantumCircuit(qr, cr, name='some_name') #  and dont't forget to put a name
        circuit.h(qr[1])                                   #  (it will be used to save the pickle
        circuit.cx(qr[1], qr[2])                           #
        circuit.measure(qr, cr)                            #

        result = transpile(circuit, self.create_backend(), coupling_map=coupling_map,
                           pass_manager=self.create_passmanager(coupling_map))
        self.assertResult(result, circuit.name)

[delapuente]

Thank you for the changes, @1ucian0. I think we are almost there. How do you plan to remove the old pickles in case the name of the circuit changes for some reason?

[delapuente]

Hey, @1ucian0, what about removing the pickles that are no longer needed after regenerating?

[1ucian0]

done in 0d65f25

[delapuente]

Looks great. My only concern is when to regenerate, I know there are plans for including a developer guide about the transpiler and passes and we can include information about it there (#83). Another option is to add some information to the CONTRIBUTING.rst guidelines, in the same way we did when we updated the test options (#1832).