Fix cases of inconsistent namespacing in tests
Sometimes e.g. Series and pd.Series are used
in the same test file. This fixes some of these
cases, generally by using the explicitly
imported class.

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