TST: Skipif decorator for matplotlib #18190
- [X] progress towards #18190
- [X] tests added / passed
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

The behavior here is slightly different than before when the function was used at the module level. Previously, an entire module being skipped would count as one skip entry. Now with the decorator, each test case within the module gets counted as a skip.