TST/CLN: declass smaller test files in tests\io\excel
xref https://github.com/pandas-dev/pandas/pull/26755#pullrequestreview-247416090

the larger files `pandas/tests/io/excel/test_readers.py` and `pandas/tests/io/excel/test_writers.py` have been left for separate PRs

any idiom changes other than required to declass are also omitted to make the diff easier to check.
@WillAyd  # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26764?src=pr&el=h1) Report
> Merging [#26764](https://codecov.io/gh/pandas-dev/pandas/pull/26764?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/0f3e8e8bd2118d56c7febe5d6081c5058cb22be7?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/26764/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/26764?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #26764      +/-   ##
==========================================
- Coverage   91.71%    91.7%   -0.01%     
==========================================
  Files         178      178              
  Lines       50740    50740              
==========================================
- Hits        46538    46533       -5     
- Misses       4202     4207       +5
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.3% <ø> (ø)` | :arrow_up: |
| #single | `41.21% <ø> (-0.11%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/26764?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/26764/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `78.94% <0%> (-10.53%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/26764/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `96.88% <0%> (-0.12%)` | :arrow_down: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/26764/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `90.84% <0%> (-0.11%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26764?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/26764?src=pr&el=footer). Last update [0f3e8e8...d9472b2](https://codecov.io/gh/pandas-dev/pandas/pull/26764?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26764?src=pr&el=h1) Report
> Merging [#26764](https://codecov.io/gh/pandas-dev/pandas/pull/26764?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/0f3e8e8bd2118d56c7febe5d6081c5058cb22be7?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/26764/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/26764?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #26764      +/-   ##
==========================================
- Coverage   91.71%    91.7%   -0.01%     
==========================================
  Files         178      178              
  Lines       50740    50740              
==========================================
- Hits        46538    46533       -5     
- Misses       4202     4207       +5
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.3% <ø> (ø)` | :arrow_up: |
| #single | `41.21% <ø> (-0.11%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/26764?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/26764/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `78.94% <0%> (-10.53%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/26764/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `96.88% <0%> (-0.12%)` | :arrow_down: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/26764/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `90.84% <0%> (-0.11%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26764?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/26764?src=pr&el=footer). Last update [0f3e8e8...d9472b2](https://codecov.io/gh/pandas-dev/pandas/pull/26764?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @simonjayhawkins these comments can be done as a followup (or make an issue) if you prefer > @simonjayhawkins these comments can be done as a followup (or make an issue) if you prefer

OK. AFK for a while now anyway.
 thanks @simonjayhawkins  > @simonjayhawkins these comments can be done as a followup (or make an issue) if you prefer

i think i'll open an issue as well, as there are comments from this and several of the recent test_excel cleanup PRs to be considered as follow-ups and it'll be easier to track.