split off scalar tests to submodules
Spun off from #19738, implements test_arithmetic for timedeltas, _moves_ and organizes tests without changing anything in the non-test code.

A handful of xfail tests are implemented (these are fixed in #19378)
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19752?src=pr&el=h1) Report
> Merging [#19752](https://codecov.io/gh/pandas-dev/pandas/pull/19752?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/c1f0c63191e318981b5a244a3f240090df63f1d0?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/19752/graphs/tree.svg?height=150&width=650&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/19752?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #19752      +/-   ##
==========================================
+ Coverage    91.6%    91.6%   +<.01%     
==========================================
  Files         150      150              
  Lines       48864    48864              
==========================================
+ Hits        44761    44762       +1     
+ Misses       4103     4102       -1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.98% <ø> (ø)` | :arrow_up: |
| #single | `41.77% <ø> (+0.01%)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/19752?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/ops.py](https://codecov.io/gh/pandas-dev/pandas/pull/19752/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvb3BzLnB5) | `96.62% <0%> (+0.16%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19752?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/19752?src=pr&el=footer). Last update [c1f0c63...6d45594](https://codecov.io/gh/pandas-dev/pandas/pull/19752?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 moved the rest of the scalar files. ping on green. @jbrockmendel you will need to rebase anything that involves scalar tests