Split and parametrize test_operators
Orthogonal to #19166 since this does not touch `TestTimedeltaSeriesArithmetic`.

Fix (some) flake8 complaints about 1-letter variable names.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19173?src=pr&el=h1) Report
> Merging [#19173](https://codecov.io/gh/pandas-dev/pandas/pull/19173?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/787ab5556f23d2e10b798d2e58e66789d79fd4d5?src=pr&el=desc) will **decrease** coverage by `0.02%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/19173/graphs/tree.svg?height=150&width=650&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/19173?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #19173      +/-   ##
==========================================
- Coverage   91.55%   91.53%   -0.03%     
==========================================
  Files         147      147              
  Lines       48812    48812              
==========================================
- Hits        44690    44678      -12     
- Misses       4122     4134      +12
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.9% <ø> (-0.03%)` | :arrow_down: |
| #single | `41.7% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/19173?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/19173/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `65.22% <0%> (-1.74%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19173?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/19173?src=pr&el=footer). Last update [787ab55...c3cd064](https://codecov.io/gh/pandas-dev/pandas/pull/19173?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 ping lgtm. needs a rebase as I just merged a pr fixing some delta things. png on green. ping thanks. next PR should be to split operators tests up into sub-files.