TST: move and reformat latex_na_rep test
- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Move test to a more suitable location (``class TestToLatexFormatters``)
and reformat using ``_dedent`` and raw string for better readability.