REF/TST: Add more pytest idiom to scalar/test_nat
Hello @gfyoung! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/tests/scalar/test_nat.py`](https://github.com/gfyoung/pandas/blob/3cafd3f3214999da6dc17a0aa298dd55a6372e2d/pandas/tests/scalar/test_nat.py) !

 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24120?src=pr&el=h1) Report
> Merging [#24120](https://codecov.io/gh/pandas-dev/pandas/pull/24120?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/b8413746d9d4c4ba6f7a1e694eabf7351f22b1a1?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/24120/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24120?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #24120      +/-   ##
==========================================
- Coverage    92.2%    92.2%   -0.01%     
==========================================
  Files         162      162              
  Lines       51701    51701              
==========================================
- Hits        47672    47671       -1     
- Misses       4029     4030       +1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.6% <ø> (ø)` | :arrow_up: |
| #single | `43.02% <ø> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/24120?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/json/json.py](https://codecov.io/gh/pandas-dev/pandas/pull/24120/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2pzb24vanNvbi5weQ==) | `92.61% <0%> (-0.48%)` | :arrow_down: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/24120/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `87.51% <0%> (+0.09%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24120?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/24120?src=pr&el=footer). Last update [b841374...3d62025](https://codecov.io/gh/pandas-dev/pandas/pull/24120?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks @gfyoung 