MAINT: Remove assertIsNotNone from testing
Title is self-explanatory.

Also removes some vestigial `assertIsNot` calls missed in #16024.

Partially addresses #15990.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16027?src=pr&el=h1) Report
> Merging [#16027](https://codecov.io/gh/pandas-dev/pandas/pull/16027?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/5146b5971324009873cf1a54c8f0828de63874ae?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/16027/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&height=150&width=650)](https://codecov.io/gh/pandas-dev/pandas/pull/16027?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #16027      +/-   ##
==========================================
- Coverage   90.99%   90.99%   -0.01%     
==========================================
  Files         153      153              
  Lines       50471    50469       -2     
==========================================
- Hits        45928    45926       -2     
  Misses       4543     4543
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.76% <ø> (-0.01%)` | :arrow_down: |
| #single | `40.43% <ø> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/16027?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/16027?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `80.73% <ø> (-0.04%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16027?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/16027?src=pr&el=footer). Last update [5146b59...813fac3](https://codecov.io/gh/pandas-dev/pandas/pull/16027?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @gfyoung Thanks!

If you would do further PRs like this, could you leave comments as is? (if it is just capitalisation or punctuation, the diff is already large enough ;-))