CLN: Consistent pandas.util.testing imports in pandas/tests/series
Part of #29272

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


lgtm Merge conflicts are going to pile up if we dont merge this promptly, merging. Thanks @SaturnFromTitan 