TST: Split test_excel.py into sub test files #24472
- [ ] closes #24472 and update to #24749
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

In brief, I have tried to split the quite large `test_excel.py` test file into logical chunks. I have done this by first splitting by reading and writing, and then by engine.
