Enable ignored test in test_fake_backends

[iuliazidaru]

What is the expected enhancement?

While working on #3905 I saw a test which is skipped until Qiskit/qiskit-aer#741 is fixed. Maybe is should be checked again.
https://github.com/Qiskit/qiskit-terra/blob/main/test/python/mock/test_fake_backends.py

    @unittest.skip("Skipped until qiskit-aer#741 is fixed and released")
    @unittest.skipUnless(HAS_AER, "qiskit-aer is required to run this test")
    def test_transpile_schedule_and_assemble(self):
        """Test transpile, schedule and assemble on generated backend."""
        qc = get_test_circuit()

[jakelishman]

Since ConfigurableFakeBackend was removed in #11568, I think this issue is obsolete now. Please feel free to re-open if there's more to discuss.