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.
can you just confirm that before & after test counts are the same for latex. "Added" tests:
```
$ git diff e260f58^..8815c7f | grep "^+.*def test_" | wc -l
44
```

"Deleted" tests:
```
$ git diff e260f58^..8815c7f | grep "^-.*def test_" | wc -l
43
```

One extra test was added: ``test_to_latex_column_format_just_works`` as a result of splitting ``test_to_latex_column_format``.
Originally ``test_to_latex_column_format`` basically comprised two tests: one to check that column format just works without failing for dataframe ``float_frame`` (fixture) and the second one to test result vs expected for small dataframe. I extracted the test ``_just_work`` for the ``float_frame``.
 thanks @ivanovmg 