Include tests for pd.NA and pd.NaT in tests/base/test_ops.py

[SaturnFromTitan]

This is a follow-up from #32281, specifically on this comment from jreback.

test_unique_null, test_nunique_null and test_value_counts_null should be tested for pd.NA and pd.NaT as well, ideally using the nulls_fixture from pandas/conftest.py.

As discovered/discussed in the mentioned PR, they would xfail for now. It's something that should be tested and ideally fixed though.

[mroeschke]

I think we have coverage of this on other tests now so closing