TST: move .str-test to strings.py & parametrize it; precursor to #23582
- [x] precursor to #23582 and #23167
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

In #23582, I'm testing all string methods on all inferred dtypes (which uncovered lots of bugs that will be fixed by #23167 and follow-ups).

@jreback required #23582 to be broken up further, so here I'm moving one existing test that effectively uses the same idea to the (IMO) correct module (it's about testing the `.str`-accessor after all), where the `any_string_method` fixture can then be used for both the test I'm moving in this PR, and the tests I'm introducing in #23582.

It makes sense to have this fixture that's directly related to `.str` in the appropriate module, and not pollute the general `pandas/conftest.py` with it, that's mainly why I'm moving the test.
