TST: GH18498 - Split test_pytables into sub-modules
- [x] closes #18498 
- [x] tests moved / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Moved test classes for pytables into sub-modules.
Moved hdf test data files needed into sub-module.

I did not change any tests, just broke up the classes into separate modules.