REF: rearrange test_to_latex.py
- [ ] closes #xxxx
- [ ] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Rearranged tests in ``test_to_latex.py`` to the corresponding classes.
Sorry for the messy diffs.
All I did (except to small commits cd92db9 and 2b66e2f) was rearrangement of the tests.