CLN: Simplify groupby head/tail tests
- [ ] closes #xxxx
- [ ] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Going to add axis=1 tests as part of resolution of #9772, simplifying test first.
thanks!
