TST: Add test for groupby sum of large ints
Title is self-explanatory.  Closes #14758.
lgtm. ping on green. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16671?src=pr&el=h1) Report
> Merging [#16671](https://codecov.io/gh/pandas-dev/pandas/pull/16671?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/0281886fbdf0d1837c2af08af15949c1d98bf612?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/16671/graphs/tree.svg?src=pr&token=eZ4WkYLtcO&height=150&width=650)](https://codecov.io/gh/pandas-dev/pandas/pull/16671?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #16671   +/-   ##
=======================================
  Coverage   90.92%   90.92%           
=======================================
  Files         161      161           
  Lines       49272    49272           
=======================================
  Hits        44803    44803           
  Misses       4469     4469
```

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


------

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> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16671?src=pr&el=footer). Last update [0281886...6ed6f4e](https://codecov.io/gh/pandas-dev/pandas/pull/16671?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @jreback : All green and ready to merge. thanks!