Support fixtures and pytest.mark.parametrize with gen_test

[jrbourbeau]

In #4958, @gjoseph92 added pytest fixture and parametrization support to our distributed.utils_test.gen_cluster decorator. We should do something similar for distributed.utils_test.gen_test

The goal would be to support use cases like

@pytest.mark.parametrize("foo", [True])
@gen_test()
async def test_gen_test_parametrized(foo):
    assert foo is True

which currently fail with

______________________________________ ERROR collecting distributed/tests/test_utils.py _______________________________________
In test_func: function uses no argument 'foo'
=================================================== short test summary info ===================================================
ERROR distributed/tests/test_utils.py