CLN: Removed class in pandas/tests/series/test_validate.py
I've seen quite a few test files that only use a class for historic reasons. As functions are easier to comprehend I think it makes sense to move in this direction.

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

