Test Decorators and Better Pytest Integration in 'test_excel'
- [x] closes #19816 
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Once I started looking at it, it made sense to make the module more "pytest compatible". There was a lot of skipping being done imperatively that seemed better suited to using class-level decorators, so as I moved some of that around I also replaced the setup/teardown mechanism of the "Excel Writing" tests with a pytest fixture