TST: Split and simplify test_value_counts_unique_nunique
closes #32205, closes #32220

- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`


The failing CI is unrelated to these changes @jreback merged master to resolve conflicts thanks @SaturnFromTitan 

one other followup on this. I think we should split out the tests you just changed (e.g. unique, value_counts) into a test_unique.py module; happy to do this generally as well (though should be separate from a refactorting PR). Will do. I'll wait until #32311 is merged though as I think it should go into the test_uniques.py as well