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

Refactor/clean-up ``test_to_latex.py``.

- Split big test functions with multiple assertions into multiple functions
- Make readable expected strings by first indenting for the good visual appearance and then dedenting by the leading whitespace for the assertion.