TST (string dtype): clean-up xpasssing tests with future string dtype
Follow-up PR on https://github.com/pandas-dev/pandas/pull/59320, doing some superficial clean-up of the tests (largely removing some xfails that were actually passing in the meantime)

xref https://github.com/pandas-dev/pandas/issues/54792
The failures with python-dev / numpy-dev seem unrelated.