CLN: Cleanup tests for .rank()
Moves `rank` tests for `DataFrame` and `Series` to respective test suites.

1. Created new `pandas/tests/series/test_rank` and `pandas/tests/frame/test_rank`
2. Moved tests from `pandas/tests/test_stats` to `series` and `frame` version of `test_rank`
3. Moved `rank` tests from `frame/test_analytics` to `frame/test_rank`
4. Moved `rank` tests from `series/test_analytics` to `series/test_analytics`

 - [X] closes #15640
 - [x] tests added / passed
 - [X] passes ``git diff upstream/master | flake8 --diff``
 - [ ] whatsnew entry
