TST verify groupby doesn't alter unit64s to floats #30859
- [x] closes #30859 
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

Logic of the test is to check that the value hasn't changes and that the dtype remains uint64.
thanks @TAJD 