TST: Add test for groupby mean of ints
Title is self-explanatory.

Closes #11199.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16670?src=pr&el=h1) Report
> Merging [#16670](https://codecov.io/gh/pandas-dev/pandas/pull/16670?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/18a428da81f7fe0f41f8ec78f03a385b731bb6a3?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/16670/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&height=150&width=650)](https://codecov.io/gh/pandas-dev/pandas/pull/16670?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #16670   +/-   ##
=======================================
  Coverage   90.93%   90.93%           
=======================================
  Files         161      161           
  Lines       49269    49269           
=======================================
  Hits        44802    44802           
  Misses       4467     4467
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.69% <ø> (ø)` | :arrow_up: |
| #single | `40.22% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16670?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/16670?src=pr&el=footer). Last update [18a428d...8802f44](https://codecov.io/gh/pandas-dev/pandas/pull/16670?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16670?src=pr&el=h1) Report
> Merging [#16670](https://codecov.io/gh/pandas-dev/pandas/pull/16670?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/18a428da81f7fe0f41f8ec78f03a385b731bb6a3?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/16670/graphs/tree.svg?src=pr&token=eZ4WkYLtcO&height=150&width=650)](https://codecov.io/gh/pandas-dev/pandas/pull/16670?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #16670   +/-   ##
=======================================
  Coverage   90.93%   90.93%           
=======================================
  Files         161      161           
  Lines       49269    49269           
=======================================
  Hits        44802    44802           
  Misses       4467     4467
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.69% <ø> (ø)` | :arrow_up: |
| #single | `40.22% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16670?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/16670?src=pr&el=footer). Last update [18a428d...8802f44](https://codecov.io/gh/pandas-dev/pandas/pull/16670?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 lgtm. if you'd have a look at the xref issues (which are closed):

xref #15091
xref #3707

to see if we want to add any tests from those. > to see if we want to add any tests from those.

Why not, if possible? :smile:  I can add those as separate PR's. ok then (FYI sometimes I have xref duplicate issues that have potentially other tests). yes a new PR is fine. @jreback : Unfortunately, both issues are still valid on `master`.  They actually were not addressed by the fixing of #11199 as you had imagined. 😢  hmm can u create a new issue with he simplest repro possible for both of those (and cited the old)
there is lots of chatter on those @jreback #3707 has been resurrected as #16674, but with a slightly broader focus.