TST: mark all excel tests as slow
- [ ] xref #38091
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Marked all excel-related tests as slow.
There are a lot of time for setup and teardown for every test.
Overall, moving these tests to "slow" category may probably save about 5 minutes of test time for "not slow" pipelines.