TST: Fix excel test discovery
closes #16477 

Locally:

```
======================================== 26 failed, 358 passed, 48 skipped, 1 xfailed, 61 warnings in 20.85 seconds ========================================
```
oh, so these were not running? hope they pass! # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16478?src=pr&el=h1) Report
> Merging [#16478](https://codecov.io/gh/pandas-dev/pandas/pull/16478?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/6cbd5589fd005f987381bacbaa79e96950cd5dcb?src=pr&el=desc) will **increase** coverage by `0.39%`.
> The diff coverage is `83.33%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/16478/graphs/tree.svg?token=eZ4WkYLtcO&width=650&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/16478?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #16478      +/-   ##
==========================================
+ Coverage   90.42%   90.81%   +0.39%     
==========================================
  Files         161      161              
  Lines       51029    51038       +9     
==========================================
+ Hits        46144    46352     +208     
+ Misses       4885     4686     -199
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.65% <83.33%> (+0.39%)` | :arrow_up: |
| #single | `40.16% <16.66%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/16478?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/excel.py](https://codecov.io/gh/pandas-dev/pandas/pull/16478?src=pr&el=tree#diff-cGFuZGFzL2lvL2V4Y2VsLnB5) | `80.49% <83.33%> (+18.17%)` | :arrow_up: |
| [pandas/core/series.py](https://codecov.io/gh/pandas-dev/pandas/pull/16478?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc2VyaWVzLnB5) | `94.9% <0%> (+0.18%)` | :arrow_up: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/16478?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `80.98% <0%> (+0.19%)` | :arrow_up: |
| [pandas/io/parsers.py](https://codecov.io/gh/pandas-dev/pandas/pull/16478?src=pr&el=tree#diff-cGFuZGFzL2lvL3BhcnNlcnMucHk=) | `95.66% <0%> (+0.34%)` | :arrow_up: |
| [pandas/io/formats/excel.py](https://codecov.io/gh/pandas-dev/pandas/pull/16478?src=pr&el=tree#diff-cGFuZGFzL2lvL2Zvcm1hdHMvZXhjZWwucHk=) | `96.65% <0%> (+22.4%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16478?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/16478?src=pr&el=footer). Last update [6cbd558...2b4a20a](https://codecov.io/gh/pandas-dev/pandas/pull/16478?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/16478?src=pr&el=h1) Report
> Merging [#16478](https://codecov.io/gh/pandas-dev/pandas/pull/16478?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/e81f3cc30725443a1d36ec27279f3fec4eed60ec?src=pr&el=desc) will **increase** coverage by `0.38%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/16478/graphs/tree.svg?height=150&src=pr&token=eZ4WkYLtcO&width=650)](https://codecov.io/gh/pandas-dev/pandas/pull/16478?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #16478      +/-   ##
==========================================
+ Coverage   90.43%   90.82%   +0.38%     
==========================================
  Files         161      161              
  Lines       51045    51038       -7     
==========================================
+ Hits        46161    46353     +192     
+ Misses       4884     4685     -199
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.65% <100%> (+0.38%)` | :arrow_up: |
| #single | `40.16% <28.57%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/16478?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/excel.py](https://codecov.io/gh/pandas-dev/pandas/pull/16478?src=pr&el=tree#diff-cGFuZGFzL2lvL2V4Y2VsLnB5) | `80.64% <100%> (+18.32%)` | :arrow_up: |
| [pandas/core/window.py](https://codecov.io/gh/pandas-dev/pandas/pull/16478?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvd2luZG93LnB5) | `96.24% <0%> (-0.24%)` | :arrow_down: |
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/16478?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `92.13% <0%> (-0.04%)` | :arrow_down: |
| [pandas/core/ops.py](https://codecov.io/gh/pandas-dev/pandas/pull/16478?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvb3BzLnB5) | `91.68% <0%> (-0.02%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/16478?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.69% <0%> (ø)` | :arrow_up: |
| [pandas/core/series.py](https://codecov.io/gh/pandas-dev/pandas/pull/16478?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc2VyaWVzLnB5) | `94.9% <0%> (+0.18%)` | :arrow_up: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/16478?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `80.98% <0%> (+0.19%)` | :arrow_up: |
| [pandas/io/parsers.py](https://codecov.io/gh/pandas-dev/pandas/pull/16478?src=pr&el=tree#diff-cGFuZGFzL2lvL3BhcnNlcnMucHk=) | `95.66% <0%> (+0.32%)` | :arrow_up: |
| [pandas/io/feather\_format.py](https://codecov.io/gh/pandas-dev/pandas/pull/16478?src=pr&el=tree#diff-cGFuZGFzL2lvL2ZlYXRoZXJfZm9ybWF0LnB5) | `87.87% <0%> (+2.16%)` | :arrow_up: |
| ... and [1 more](https://codecov.io/gh/pandas-dev/pandas/pull/16478?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16478?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/16478?src=pr&el=footer). Last update [e81f3cc...a3bcbd1](https://codecov.io/gh/pandas-dev/pandas/pull/16478?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 ahh this is fixing up after #16442?  Yeah, 11bed216c3021e19618a9c65e6c0430cdf887826 fixes that. Would have been an unfortunate breakage :/ Ugh. That random matplotlib failure is spreading: https://ci.appveyor.com/project/pandas-dev/pandas/build/1.0.2225/job/6lg79jfhas0l9qiw#L1128 (I hadn't seen it on windows yet)

@jreback ok to merge? Or can you restart appveyor? yep @TomAugspurger You will have to split this commit if you want to backport this PR, as the sheetname change is only targetted for 0.21.0 > @TomAugspurger You will have to split this commit if you want to backport this PR, as the sheetname change is only targetted for 0.21.0

I hadn't thought of that... I think I'll not backport it then, unless there are objections. It's only about some excel test not running on the 0.20.x branch? That sounds OK to me to not backport if we don't have any excel related changes in 0.20.x

