Split test_excel.py (#24472)
Split tests/io/test_excel.py into two files, base.py and xlrd.py located in tests/io/excel/. The previous file was very large and a logical split has been applied to separate the `xlrd` tests.

- [ ] closes #24472 
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
