TST: don't use global fixture in the base extension tests
Follow-up on https://github.com/pandas-dev/pandas/pull/37867. In the discussion about using this fixture, I forgot that we should not use global pandas fixtures in those tests, but only the ones that are defined in the local /pandas/tests/extension/conftest.py, otherwise this doesn't work for downstream projects (eg https://github.com/geopandas/geopandas/issues/1735). For this case it's easier to simply not use a fixture, I think (but still share the common definition of the list)