TST: Verify isin returns consistent results for passed iterables
Add tests to verify that the `Series` method `isin` returns consistent results when passed iterables with different dtypes. (See #50234.)

- [x] closes #50234
- [ ] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature
- [ ] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [x] ~Added [type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) to new arguments/methods/functions.~ (Not applicable.)
- [x] ~Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.~ (Not applicable.)

Thx @jessica-writes-code 