TST: add method/dtype coverage to str-accessor; precursor to #23167
- [x] PRECURSOR to #23167
- [x] tests added / passed / xfailed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

The effort to test all methods of the `.str`-accessor against all inferred data types in #23167 unearthed a bunch of bugs. However, that PR is getting quite unwieldy, and since @jreback much prefers single-purpose PRs (and in the spirit of test-driven development), I'm splitting off just the addition of the parametrized fixtures / tests, with *lots* of `xfails` that will be removed by #23167 and subsequent PRs.
