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

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15658?src=pr&el=h1) Report
> Merging [#15658](https://codecov.io/gh/pandas-dev/pandas/pull/15658?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/5eac08a4f7fb2416fd7d3470e111d203a9a23feb?src=pr&el=desc) will **decrease** coverage by `0.02%`.
> The diff coverage is `n/a`.


```diff
@@            Coverage Diff             @@
##           master   #15658      +/-   ##
==========================================
- Coverage   91.03%   91.01%   -0.03%     
==========================================
  Files         143      143              
  Lines       49382    49382              
==========================================
- Hits        44957    44945      -12     
- Misses       4425     4437      +12
```


| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/15658?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/compare/5eac08a4f7fb2416fd7d3470e111d203a9a23feb...d0a2abc659abf65541f3ec6fd8e1f3b4a08edc1b?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :x: |
| [pandas/tools/merge.py](https://codecov.io/gh/pandas-dev/pandas/compare/5eac08a4f7fb2416fd7d3470e111d203a9a23feb...d0a2abc659abf65541f3ec6fd8e1f3b4a08edc1b?src=pr&el=tree#diff-cGFuZGFzL3Rvb2xzL21lcmdlLnB5) | `93.61% <0%> (-0.34%)` | :x: |
| [pandas/core/common.py](https://codecov.io/gh/pandas-dev/pandas/compare/5eac08a4f7fb2416fd7d3470e111d203a9a23feb...d0a2abc659abf65541f3ec6fd8e1f3b4a08edc1b?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tbW9uLnB5) | `90.96% <0%> (-0.34%)` | :x: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/compare/5eac08a4f7fb2416fd7d3470e111d203a9a23feb...d0a2abc659abf65541f3ec6fd8e1f3b4a08edc1b?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.87% <0%> (-0.1%)` | :x: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15658?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15658?src=pr&el=footer). Last update [5eac08a...d0a2abc](https://codecov.io/gh/pandas-dev/pandas/compare/5eac08a4f7fb2416fd7d3470e111d203a9a23feb...d0a2abc659abf65541f3ec6fd8e1f3b4a08edc1b?el=footer&src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments). thanks!