TST: Add tests string series min max
Add tests for max and min aggregation at string series

In line with #31746

- [X] closes #31746
- [X] tests added / passed
- [X] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit) for how to run them

thanks @NumberPiOso 