Regression in testdir fixture

You can see further details in our Dependabot PR: https://github.com/aio-libs/aiohttp/pull/8704

Basically, upgrading from 8.1.1 seems to have caused one of our tests to fail (I think there was another regression in 8.2 which has been fixed, so not sure if this was introduced in 8.2 or 8.3). The code is:

def test_testcase_no_app(
    testdir: pytest.Testdir, loop: asyncio.AbstractEventLoop
) -> None:
    testdir.makepyfile(
        """
        from aiohttp.test_utils import AioHTTPTestCase


        class InvalidTestCase(AioHTTPTestCase):
            def test_noop(self) -> None:
                pass
        """
    )
    result = testdir.runpytest()
    result.stdout.fnmatch_lines(["*TypeError*"])

The logged output from pytest says that it's collected 0 items. My first guess is that the default filename may have changed, thus causing pytest to not find any test files? According to the documentation, I believe the default filename should be test_testcase_no_app.py.

Start with the test_testcase_no_app fixture and the linked Dependabot PR #8704, then run the shown test to confirm why pytest collects zero items after the dependency upgrade. Compare the expected default filename, test_testcase_no_app.py, with the observed behavior; done means the regression is identified and the test again reaches the expected TypeError assertion.