TST: fix casting extension tests for external users
cc @simonjayhawkins fixtures defined by pandas are not known when inheriting the tests as external package. There are a bunch of fixtures that external packages are expected to define in `pandas/tests/extension/conftest.py`, but I think for the string dtype here this is not worth it to expect downstream users to define it themselves (certainly given that it is still going to change)
Failure is the flaky ResourceWarning