TST : add test for groupby aggregation dtype
- [ ] closes #32793
- [ ] tests added / passed
- [ ] 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
- [ ] whatsnew entry

https://github.com/pandas-dev/pandas/pull/43915/checks?check_run_id=3885731836 fail, I check but I dont understand why in my code impact that. thanks @OlivierCavadenti 