Remove SharedItems from test_excel
Pretty big step towards logically partitioning tests in this module - replaced SharedItems and instance attributes with fixtures. Should make subsequent test reorg easier

@simonjayhawkins 
@simonjayhawkins thanks for the thorough review - definitely making this PR a lot better. Latest push I think should address your callouts # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26579?src=pr&el=h1) Report
> Merging [#26579](https://codecov.io/gh/pandas-dev/pandas/pull/26579?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/101370645d13e1d0f256f367f4ef56a8329b56b6?src=pr&el=desc) will **decrease** coverage by `50.07%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/26579/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/26579?src=pr&el=tree)

```diff
@@             Coverage Diff             @@
##           master   #26579       +/-   ##
===========================================
- Coverage   91.87%    41.8%   -50.08%     
===========================================
  Files         174      174               
  Lines       50692    50692               
===========================================
- Hits        46575    21190    -25385     
- Misses       4117    29502    +25385
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `?` | |
| #single | `41.8% <ø> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/26579?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/formats/latex.py](https://codecov.io/gh/pandas-dev/pandas/pull/26579/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2Zvcm1hdHMvbGF0ZXgucHk=) | `0% <0%> (-100%)` | :arrow_down: |
| [pandas/io/sas/sas\_constants.py](https://codecov.io/gh/pandas-dev/pandas/pull/26579/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL3Nhcy9zYXNfY29uc3RhbnRzLnB5) | `0% <0%> (-100%)` | :arrow_down: |
| [pandas/core/groupby/categorical.py](https://codecov.io/gh/pandas-dev/pandas/pull/26579/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS9jYXRlZ29yaWNhbC5weQ==) | `0% <0%> (-100%)` | :arrow_down: |
| [pandas/tseries/plotting.py](https://codecov.io/gh/pandas-dev/pandas/pull/26579/diff?src=pr&el=tree#diff-cGFuZGFzL3RzZXJpZXMvcGxvdHRpbmcucHk=) | `0% <0%> (-100%)` | :arrow_down: |
| [pandas/tseries/converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/26579/diff?src=pr&el=tree#diff-cGFuZGFzL3RzZXJpZXMvY29udmVydGVyLnB5) | `0% <0%> (-100%)` | :arrow_down: |
| [pandas/io/formats/html.py](https://codecov.io/gh/pandas-dev/pandas/pull/26579/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2Zvcm1hdHMvaHRtbC5weQ==) | `0% <0%> (-99.37%)` | :arrow_down: |
| [pandas/io/sas/sas7bdat.py](https://codecov.io/gh/pandas-dev/pandas/pull/26579/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL3Nhcy9zYXM3YmRhdC5weQ==) | `0% <0%> (-91.16%)` | :arrow_down: |
| [pandas/io/sas/sas\_xport.py](https://codecov.io/gh/pandas-dev/pandas/pull/26579/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL3Nhcy9zYXNfeHBvcnQucHk=) | `0% <0%> (-90.1%)` | :arrow_down: |
| [pandas/core/sparse/scipy\_sparse.py](https://codecov.io/gh/pandas-dev/pandas/pull/26579/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc3BhcnNlL3NjaXB5X3NwYXJzZS5weQ==) | `10.14% <0%> (-89.86%)` | :arrow_down: |
| [pandas/core/tools/numeric.py](https://codecov.io/gh/pandas-dev/pandas/pull/26579/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvdG9vbHMvbnVtZXJpYy5weQ==) | `10.14% <0%> (-89.86%)` | :arrow_down: |
| ... and [128 more](https://codecov.io/gh/pandas-dev/pandas/pull/26579/diff?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26579?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/26579?src=pr&el=footer). Last update [1013706...5d0b631](https://codecov.io/gh/pandas-dev/pandas/pull/26579?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/26579?src=pr&el=h1) Report
> Merging [#26579](https://codecov.io/gh/pandas-dev/pandas/pull/26579?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/101370645d13e1d0f256f367f4ef56a8329b56b6?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/26579/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/26579?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #26579      +/-   ##
==========================================
- Coverage   91.87%   91.86%   -0.02%     
==========================================
  Files         174      174              
  Lines       50692    50692              
==========================================
- Hits        46575    46569       -6     
- Misses       4117     4123       +6
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.4% <ø> (-0.01%)` | :arrow_down: |
| #single | `41.78% <ø> (-0.09%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/26579?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/26579/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `78.94% <0%> (-10.53%)` | :arrow_down: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/26579/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `90.6% <0%> (-0.22%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/26579/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97% <0%> (-0.12%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26579?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/26579?src=pr&el=footer). Last update [1013706...199909a](https://codecov.io/gh/pandas-dev/pandas/pull/26579?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks @WillAyd 