Tests for fixed issues in Series[datetime64]
- [x] closes #12534, closes #19071
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

@jbrockmendel : To properly close #12534, you should also add documentation per the discussion. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19072?src=pr&el=h1) Report
> Merging [#19072](https://codecov.io/gh/pandas-dev/pandas/pull/19072?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/4a8496b2bf02df521b6b7f15843c515545a3d192?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/19072/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/19072?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #19072      +/-   ##
==========================================
- Coverage   91.54%   91.51%   -0.03%     
==========================================
  Files         148      148              
  Lines       48804    48804              
==========================================
- Hits        44676    44664      -12     
- Misses       4128     4140      +12
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.89% <ø> (-0.03%)` | :arrow_down: |
| #single | `41.63% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/19072?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/19072/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `65.22% <0%> (-1.74%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19072?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/19072?src=pr&el=footer). Last update [4a8496b...b2db46f](https://codecov.io/gh/pandas-dev/pandas/pull/19072?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 > To properly close #12534, you should also add documentation per the discussion

@gfyoung Can you expand on what you have in mind here? > were these fixed by a previous PR? can you reference it

For one of the tests I know it was fixed by #19024.  For the other I'm not sure when it got fixed (but certainly no later than 19024) thanks, if these issues are not in the whatsnew pls add (new PR).