parameterize tests in scalar/timedelta
- [x] closes https://github.com/pandas-dev/pandas/issues/20426
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20428?src=pr&el=h1) Report
> Merging [#20428](https://codecov.io/gh/pandas-dev/pandas/pull/20428?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/01882ba5b4c21b0caf2e6b9279fb01967aa5d650?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/20428/graphs/tree.svg?height=150&width=650&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/20428?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #20428      +/-   ##
==========================================
- Coverage    91.8%   91.77%   -0.03%     
==========================================
  Files         152      152              
  Lines       49215    49215              
==========================================
- Hits        45181    45169      -12     
- Misses       4034     4046      +12
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.16% <ø> (-0.03%)` | :arrow_down: |
| #single | `41.84% <ø> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/20428?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/20428/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `65.07% <0%> (-1.74%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20428?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/20428?src=pr&el=footer). Last update [01882ba...e5df1dd](https://codecov.io/gh/pandas-dev/pandas/pull/20428?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thans @minggli 

happyily will take any parameterization tests! would take even w/o an issue