What does this PR do?

I was refactoring the Accelerate tests today and I noticed we use if .. else.. as a conditional for skipping tests based on imports. Unittest has skipUnless and skipIf, letting us simplify those decorators to be one line.

E.g.:

    if not _run_slow_tests:
        return unittest.skip("test is slow")(test_case)
    else:
        return test_case

Can be:

    return unittest.skipUnless(_run_slow_tests, "test is slow")(test_case)

(Adding you as a reviewer for this Sylvain, unsure who else should be added)