is_django_unittest gives false negatives during pytest_collection_modifyitems

`django_compat.is_django_unittest` would give false negatives during test collection. This made it hard for me to filter db tests from non-db test.

I'm not thrilled with what amounts to putting a test inside the conftest module, but I'm not sure if there's another way to test this? The `@pytest.fixture` I added was passing with the original code. Only the change to conftest exposed this problem (which I how I ran across it in the first place).