separate numeric tests so we can isolate division by zero
The upcoming fix(es) for #19322 are going to involve parametrizing a bunch of variants of division by zero.  This separates out the existing test cases into method-specific tests, some of which will be parametrized in upcoming PRs.

This PR does not change the aggregate contents of the tests.
Just implemented _most_ of the requests.  The parametrization of zeros is being put off for now because specifying all of the existing failure modes is a task unto itself. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19336?src=pr&el=h1) Report
> Merging [#19336](https://codecov.io/gh/pandas-dev/pandas/pull/19336?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/35812eaaecebeeee0ddf07dee4b583c4eea07785?src=pr&el=desc) will **increase** coverage by `0.02%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/19336/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/19336?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #19336      +/-   ##
==========================================
+ Coverage   91.67%   91.69%   +0.02%     
==========================================
  Files         148      148              
  Lines       48553    48553              
==========================================
+ Hits        44513    44523      +10     
+ Misses       4040     4030      -10
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.06% <ø> (+0.02%)` | :arrow_up: |
| #single | `41.72% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/19336?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/19336/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `83.64% <0%> (-0.21%)` | :arrow_down: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/19336/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `66.95% <0%> (+1.73%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19336?src=pr&el=continue).
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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19336?src=pr&el=footer). Last update [35812ea...4b78542](https://codecov.io/gh/pandas-dev/pandas/pull/19336?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 > as a style note, generally I would write this like

Noted for next time around. I think this should be good to go.  Will make the test-duplication discussed elsewhere easier. This is now a blocker for the ongoing centralizing-of-arithmetic tests. ping This is now a blocker for fixing series division by zero. thanks