add test cases for GroupBy.apply trivial cases 
- [x] regarding #20066
- 2 tests added, one should pass, one expected to fail.
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20081?src=pr&el=h1) Report
> Merging [#20081](https://codecov.io/gh/pandas-dev/pandas/pull/20081?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/7c14e4f14aff216be558bf5d4d2d00b4838c2360?src=pr&el=desc) will **decrease** coverage by `0.02%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/20081/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/20081?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #20081      +/-   ##
==========================================
- Coverage   91.72%    91.7%   -0.03%     
==========================================
  Files         150      150              
  Lines       49122    49137      +15     
==========================================
+ Hits        45057    45060       +3     
- Misses       4065     4077      +12
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.08% <ø> (-0.03%)` | :arrow_down: |
| #single | `41.85% <ø> (-0.02%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/20081?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/20081/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `65.07% <0%> (-1.74%)` | :arrow_down: |
| [pandas/io/html.py](https://codecov.io/gh/pandas-dev/pandas/pull/20081/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2h0bWwucHk=) | `86.6% <0%> (+0.62%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20081?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/20081?src=pr&el=footer). Last update [7c14e4f...ac6d5ae](https://codecov.io/gh/pandas-dev/pandas/pull/20081?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 slightly off topic. It would be nice to split up tests_groupby.py somewhat if you would like to tackle this.  Ideally would group some similar tests together and split out some to either existing sub-modules or some new ones. @jreback probably not right now. I've just learned to use GroupBy stuffs... and then got hit by a bug... thanks. will merge after the doc sprint. thanks @zym1010 

love for you to look into this issue (or others!)