TST: extension tests use its own fixtures
This reverts a small part of https://github.com/pandas-dev/pandas/pull/56583, for the changes made in the `pandas/tests/extension` directory

This is still a bit brittle, as there is currently no check inplace we don't change this back in the future -> https://github.com/pandas-dev/pandas/issues/56735. I was thinking we could run the tests for one of our own extension arrays outside of the pandas tests, will comment more on the issue.

Thanks @jorisvandenbossche 