[MRG] TST: Added test for groupby apply datetimeindex fix
- [x] closes #26182
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

worked on by @ezebunandu and @amy12xx

@arw2019 made the change. Let me know if you have any other comments. Thanks. Thanks @amy12xx 