TST: Cleanup test_indexing
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`

cleaned up `test_indexing` which is bit large, splitting `@slow` tests to a separate file.

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13982?src=pr) is 85.28% (diff: 100%)

> Merging [#13982](https://codecov.io/gh/pydata/pandas/pull/13982?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will not change coverage

``` diff
@@             master     #13982   diff @@
==========================================
  Files           139        139          
  Lines         50206      50206          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
  Hits          42818      42818          
  Misses         7388       7388          
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [29d9e24...3947468](https://codecov.io/gh/pydata/pandas/compare/29d9e24f4c778b0c9ebe9288bfc217808d2c6edb...3947468defcf55c815a117e9babb530e9097a5a3?src=pr)
 @sinhrks Did you just move part of the file test_indexing.py to a new file (split up)? (github won't show the diff ..)
 Yeah moved some tests to another file and fixed formats. Doesn't change tests itself. 
 thanks @sinhrks you are really putting lots of PR's up! fantastic!
