Split+Parametrize Timedelta tests
Started splitting non-contentious parts off of #19365.  This even-easier bit got split off of that.  Just organizing+splitting+parametrizing tests.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19736?src=pr&el=h1) Report
> Merging [#19736](https://codecov.io/gh/pandas-dev/pandas/pull/19736?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/718d0679600fa025872d8b66c198a6781b312d70?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/19736/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/19736?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #19736      +/-   ##
==========================================
+ Coverage    91.6%   91.61%   +<.01%     
==========================================
  Files         150      150              
  Lines       48864    48887      +23     
==========================================
+ Hits        44763    44786      +23     
  Misses       4101     4101
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.98% <ø> (ø)` | :arrow_up: |
| #single | `41.8% <ø> (+0.02%)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/19736?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/panel.py](https://codecov.io/gh/pandas-dev/pandas/pull/19736/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcGFuZWwucHk=) | `97.3% <0%> (ø)` | :arrow_up: |
| [pandas/core/ops.py](https://codecov.io/gh/pandas-dev/pandas/pull/19736/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvb3BzLnB5) | `96.74% <0%> (+0.12%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19736?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/19736?src=pr&el=footer). Last update [718d067...a852a4b](https://codecov.io/gh/pandas-dev/pandas/pull/19736?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 For now I'll update this PR to make timedeltas/test_construction.py, will move the others after #19744 (a not just-refactoring PR) goes through. This didn't get an explicit "ping on green", but pretty close.  ping. lgtm ping on green thanks!