TST: Add tests for single group
- [x] closes #13530
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`

lgtm
 ideally would add this issue number to the xref issue which actually closed this
 ## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13561?src=pr) is **84.34%**

> Merging [#13561](https://codecov.io/gh/pydata/pandas/pull/13561?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will not change coverage

``` diff
@@             master     #13561   diff @@
==========================================
  Files           138        138          
  Lines         51126      51126          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
  Hits          43122      43122          
  Misses         8004       8004          
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last updated by [f20b41e...9937e99](https://codecov.io/gh/pydata/pandas/compare/f20b41e73fdc84089289cd3999762892f85ec4b6...9937e9933e2b73e5523355ceee975eeffbe3f575)
