Centralize arithmetic tests for datetime/timedelta series
No tests were harming in the making of this PR.
Hello @jbrockmendel! Thanks for submitting the PR.

 - In the file [`pandas/tests/series/test_operators.py`](https://github.com/pandas-dev/pandas/blob/f2a8948d3d6d0ffe74d0b7312825415e918b70b4/pandas/tests/series/test_operators.py), following are the PEP8 issues :

> [Line 561:17](https://github.com/pandas-dev/pandas/blob/f2a8948d3d6d0ffe74d0b7312825415e918b70b4/pandas/tests/series/test_operators.py#L561): [E741](https://duckduckgo.com/?q=pep8%20E741) ambiguous variable name 'l'
> [Line 564:17](https://github.com/pandas-dev/pandas/blob/f2a8948d3d6d0ffe74d0b7312825415e918b70b4/pandas/tests/series/test_operators.py#L564): [E741](https://duckduckgo.com/?q=pep8%20E741) ambiguous variable name 'l'
> [Line 1390:13](https://github.com/pandas-dev/pandas/blob/f2a8948d3d6d0ffe74d0b7312825415e918b70b4/pandas/tests/series/test_operators.py#L1390): [E722](https://duckduckgo.com/?q=pep8%20E722) do not use bare except'

 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18809?src=pr&el=h1) Report
> Merging [#18809](https://codecov.io/gh/pandas-dev/pandas/pull/18809?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/b5f1e716d89487423bb12d6cc4e6da2b39184531?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18809/graphs/tree.svg?token=eZ4WkYLtcO&height=150&src=pr&width=650)](https://codecov.io/gh/pandas-dev/pandas/pull/18809?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18809      +/-   ##
==========================================
- Coverage   91.64%   91.62%   -0.02%     
==========================================
  Files         154      154              
  Lines       51430    51430              
==========================================
- Hits        47131    47122       -9     
- Misses       4299     4308       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.49% <ø> (ø)` | :arrow_up: |
| #single | `40.83% <ø> (-0.12%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18809?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/18809/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/18809/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.68% <0%> (-0.11%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18809?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/18809?src=pr&el=footer). Last update [b5f1e71...f2a8948](https://codecov.io/gh/pandas-dev/pandas/pull/18809?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks. so then you want another pass to parameterize some of these? > so then you want another pass to parameterize some of these?

Yah, plus break up by op.  Ultimately I want to make sure same set of cases get tested for the datetimelike series as for the datetimelike indexes.