WIP: TST: split up tests/plotting/test_frame.py into subdir & modules #34769
Moved `tests/plotting/test_frame.py` to `tests/plotting/frame/test_frame.py`.

Worked towards #34769 but not closing it as this has not split the test suite.

- [ ] xref #34769
- [x] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
