TST: split up tests/plotting/test_frame.py into subdir & modules #34769
- [ ] closes #34769
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

I moved the file to a new directory.
What is the best structure for this folder?