Fix mypy errors for pandas\tests\* #28926 (test_algos.py)
- [x] xref #28926
- [x] tests added / passed (verified that tests still pass)
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [N/A] whatsnew entry

Thanks @lrm25 for the PR. Nice find. Not sure this was even testing what was intended previously, so definitely speaks to annotations being useful for tests  Thanks!