TST: Regression testing for fixed issues
- [x] closes #5764
- [x] closes #7883
- [x] closes #7820
- [x] closes #8720
- [x] closes #6783
- [x] closes #7072
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
