TST: Fix test assertion
xref https://github.com/pandas-dev/pandas/pull/23304#discussion_r228458689

cc @jorisvandenbossche 
Hello @TomAugspurger! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/tests/extension/base/setitem.py`](https://github.com/TomAugspurger/pandas/blob/8540ee9eeb91ec2efc1afe3c4460b62d8e7e5e23/pandas/tests/extension/base/setitem.py) !

 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23357?src=pr&el=h1) Report
> Merging [#23357](https://codecov.io/gh/pandas-dev/pandas/pull/23357?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/cd16b91e5e33c2ad503492458827e5312f0d0659?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/23357/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/23357?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #23357   +/-   ##
=======================================
  Coverage   92.22%   92.22%           
=======================================
  Files         169      169           
  Lines       51259    51259           
=======================================
  Hits        47275    47275           
  Misses       3984     3984
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.66% <ø> (ø)` | :arrow_up: |
| #single | `42.23% <ø> (ø)` | :arrow_up: |


------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23357?src=pr&el=footer). Last update [cd16b91...8540ee9](https://codecov.io/gh/pandas-dev/pandas/pull/23357?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
