TST: Add test for skipfooter + decimal in read_csv
Title is self-explanatory.  Only for Python engine because C engine doesn't support `skipfooter` yet.

Closes #6971.

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13800?src=pr) is 85.23% (diff: 100%)

> Merging [#13800](https://codecov.io/gh/pydata/pandas/pull/13800?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will not change coverage

``` diff
@@             master     #13800   diff @@
==========================================
  Files           140        140          
  Lines         50415      50415          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
  Hits          42970      42970          
  Misses         7445       7445          
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [98c5b88...f1ded8c](https://codecov.io/gh/pydata/pandas/compare/98c5b88d6f9f7bb0afa4fbae49a045d93a1cb33f...f1ded8c26ac0c62e44c44e48de3ffb6784e4526c?src=pr)
 can you replicate the test in the associated issue as well.
 @jreback : Ah, yes, good point.  Done.
 thanks @gfyoung 
 Also got a nice, round number for this PR as a bonus! :smile:
