implement test_scalar_compat
More test organizing, this one focusing on tests.indexes.datetimes.

- continue collecting staggler arithmetic tests
- new test module for testing that DatetimeIndex.foo behaves like Timestamp.foo
- de-duplicate some tests scattered across modules
- parametrize a handful of tests
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19479?src=pr&el=h1) Report
> Merging [#19479](https://codecov.io/gh/pandas-dev/pandas/pull/19479?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/12ac43f98c6d3ce9c0c48ed75b80346881c1e4d4?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/19479/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/19479?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #19479   +/-   ##
=======================================
  Coverage   91.62%   91.62%           
=======================================
  Files         150      150           
  Lines       48728    48728           
=======================================
  Hits        44645    44645           
  Misses       4083     4083
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.99% <ø> (ø)` | :arrow_up: |
| #single | `41.74% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19479?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/19479?src=pr&el=footer). Last update [12ac43f...b9cdd99](https://codecov.io/gh/pandas-dev/pandas/pull/19479?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks!