Rewrite dict literal for files in tests/plotting/frame/test_frame_subplots.py
- [ ] xref #38138
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry 
   identify unnecessary dict call - rewrite as a literal
Rewrites as dictionary literals for the following files:

    pandas/tests/plotting/frame/test_frame_subplots.py

thanks, see my comments on your other PR; you can add more changes in a single PR. If we are getting near the end then pls indicate that (as we actually want to turn o nthe flake option).