TST: Test aggregation over arrays (#3788)
 - [x] closes #3788
 - [x] tests added / passed
 - [x] passes ``git diff upstream/master | flake8 --diff``

you are picking up the commit for the pivot table PR. this should be separate
 @mroeschke specifically, the workflow should be
- sync your master with upstream `git fetch upstream; git rebase upstream/master` from your master branch
- checkout a new branch for each change `git co -b array-agg`

You should just be able to `git rebase -i upstream/master` and delete the commit with the pivot table changes and force push.
 Ah didn't notice this picked up the commit from my other PR. Thanks for the guidance! I'll decouple it later tonight. 
 ## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/14675?src=pr) is 85.21% (diff: 100%)

> Merging [#14675](https://codecov.io/gh/pandas-dev/pandas/pull/14675?src=pr) into [master](https://codecov.io/gh/pandas-dev/pandas/branch/master?src=pr) will increase coverage by **<.01%**

``` diff
@@             master     #14675   diff @@
==========================================
  Files           143        143          
  Lines         50787      50787          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          43273      43276     +3   
+ Misses         7514       7511     -3   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [f26b049...01dce9d](https://codecov.io/gh/pandas-dev/pandas/compare/f26b049786624ed983f2718687c23e3f1adbb670...01dce9d82e7fa751d24355b9c6eb9aa6a548cb88?src=pr)
 I checked out the travis-ci build logs and it doesn't look like my change directly caused the build to fail (to my knowledge). Is there anything I need to address on my end?

EDIT: Reran the tests, and they are now passing 
 thanks!