Split test_excel into submodule
- [X] closes #24472

@simonjayhawkins 

Just moved everything as is. Some of the class structures are unnecessary now but leaving for follow ups
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26755?src=pr&el=h1) Report
> Merging [#26755](https://codecov.io/gh/pandas-dev/pandas/pull/26755?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/c7748ca057cad4a80a6c4cbf8b49c5d9f3e7ea1c?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/26755/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/26755?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #26755      +/-   ##
==========================================
- Coverage    91.7%   91.69%   -0.01%     
==========================================
  Files         179      179              
  Lines       50767    50767              
==========================================
- Hits        46555    46552       -3     
- Misses       4212     4215       +3
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.29% <ø> (ø)` | :arrow_up: |
| #single | `41.22% <ø> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/26755?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/26755/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/26755/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/26755/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `90.73% <0%> (+0.1%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26755?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/26755?src=pr&el=footer). Last update [c7748ca...0f9ceaf](https://codecov.io/gh/pandas-dev/pandas/pull/26755?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/26755?src=pr&el=h1) Report
> Merging [#26755](https://codecov.io/gh/pandas-dev/pandas/pull/26755?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/c7748ca057cad4a80a6c4cbf8b49c5d9f3e7ea1c?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/26755/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/26755?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #26755      +/-   ##
==========================================
- Coverage    91.7%   91.69%   -0.01%     
==========================================
  Files         179      178       -1     
  Lines       50767    50758       -9     
==========================================
- Hits        46555    46543      -12     
- Misses       4212     4215       +3
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.29% <ø> (-0.01%)` | :arrow_down: |
| #single | `41.21% <ø> (-0.08%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/26755?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/26755/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/26755/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `96.88% <0%> (-0.12%)` | :arrow_down: |
| [pandas/io/excel/\_\_init\_\_.py](https://codecov.io/gh/pandas-dev/pandas/pull/26755/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2V4Y2VsL19faW5pdF9fLnB5) | | |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/26755/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `90.73% <0%> (+0.1%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26755?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/26755?src=pr&el=footer). Last update [c7748ca...d2a5d3a](https://codecov.io/gh/pandas-dev/pandas/pull/26755?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Yea I think the init is a good idea to preserve the namespacing so just pushed. Also updated OP to reflect closure of issue @jreback IIUC this is just a plain split and move, no changes. your comments will be addressed in follow-ups.  ok that's fine @WillAyd I know it was a lot of work to be able to get to this point. If you now want to focus and push on with #25092 and #25427, then i'll be happy to carry-on with the clean-up and address @jreback comments.  @simonjayhawkins that would be great - thanks for offering!