TST: Clean up tests of DataFrame.sort_{index,values}
Before taking a stab at #10806, I looked at the relevant tests and they didn't make much sense to me.

This commit fixes:
- Some expected results were obtained by running code identical to what's being tested. (i.e. test could never fail)
- Removed duplicate assertions testing the same functionality both in `test_sort_index` and in `test_sort_values`.
- Switched the names `test_sort_index` and `test_sort_values` based on the (remaining) assertions being tested in their bodies.

Still confusing, unfixed:
- The `DataFrame.sort_index` docstring doesn't specify what happens when `level` is unspecified and `sort_remaining=False`. However there is a legacy test using this case: it's not clear to me what result or behavior is being tested.

First time pandas contributor, so I'd appreciate a good critical review!
- [X] tests pass
- [X] passes `git diff upstream/master | flake8 --diff`
- [x] ensure default options in docstrings
