CLN: More pytest idioms in pandas/tests/window
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

Removed some unused variables, added some fixtures for DataFrames used in `test_pairwise.py`, and utilize more `pytest.mark.parameterize`
havent looked through test_pairwise yet, but the other files all LGTM