TST: mark all plotting tests as slow
- [ ] xref https://github.com/pandas-dev/pandas/pull/37735#issuecomment-732530791
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Marked all plotting tests as slow globally at the top of each module.