Add tests for Excel styling that were missing
It seems these cases were untested by accident when styler to excel conversion was recently added.

 - [x] tests added / passed
 - [x] passes ``git diff upstream/master --name-only -- '*.py' | flake8 --diff``

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16313?src=pr&el=h1) Report
> Merging [#16313](https://codecov.io/gh/pandas-dev/pandas/pull/16313?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/81aa70c2a598f9c12d8cb56a4d758b7f213770fe?src=pr&el=desc) will **decrease** coverage by `0.04%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/16313/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/16313?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #16313      +/-   ##
==========================================
- Coverage   90.39%   90.35%   -0.05%     
==========================================
  Files         161      161              
  Lines       50863    50863              
==========================================
- Hits        45978    45956      -22     
- Misses       4885     4907      +22
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.13% <ø> (-0.03%)` | :arrow_down: |
| #single | `40.33% <ø> (-0.11%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/16313?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/16313?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/16313?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `63.23% <0%> (-1.82%)` | :arrow_down: |
| [pandas/core/common.py](https://codecov.io/gh/pandas-dev/pandas/pull/16313?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tbW9uLnB5) | `90.68% <0%> (-0.35%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/16313?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.59% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16313?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/16313?src=pr&el=footer). Last update [81aa70c...962df50](https://codecov.io/gh/pandas-dev/pandas/pull/16313?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/16313?src=pr&el=h1) Report
> Merging [#16313](https://codecov.io/gh/pandas-dev/pandas/pull/16313?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/81aa70c2a598f9c12d8cb56a4d758b7f213770fe?src=pr&el=desc) will **decrease** coverage by `0.04%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/16313/graphs/tree.svg?token=eZ4WkYLtcO&height=150&width=650&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/16313?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #16313      +/-   ##
==========================================
- Coverage   90.39%   90.35%   -0.05%     
==========================================
  Files         161      161              
  Lines       50863    50863              
==========================================
- Hits        45978    45956      -22     
- Misses       4885     4907      +22
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.13% <ø> (-0.03%)` | :arrow_down: |
| #single | `40.33% <ø> (-0.11%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/16313?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/16313?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/16313?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `63.23% <0%> (-1.82%)` | :arrow_down: |
| [pandas/core/common.py](https://codecov.io/gh/pandas-dev/pandas/pull/16313?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tbW9uLnB5) | `90.68% <0%> (-0.35%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/16313?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.59% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16313?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/16313?src=pr&el=footer). Last update [81aa70c...962df50](https://codecov.io/gh/pandas-dev/pandas/pull/16313?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks!