Add tests for Excel styling that were missing

[jnothman]

It seems these cases were untested by accident when styler to excel conversion was recently added.

• tests added / passed
• passes git diff upstream/master --name-only -- '*.py' | flake8 --diff

[codecov]

Codecov Report

Merging #16313 into master will decrease coverage by 0.04%.
The diff coverage is n/a.

@@            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%)	⬇️
#single	40.33% <ø> (-0.11%)	⬇️

Impacted Files	Coverage Δ
pandas/io/gbq.py	25% <0%> (-58.34%)	⬇️
pandas/plotting/_converter.py	63.23% <0%> (-1.82%)	⬇️
pandas/core/common.py	90.68% <0%> (-0.35%)	⬇️
pandas/core/frame.py	97.59% <0%> (-0.1%)	⬇️

---

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[codecov]

Codecov Report

Merging #16313 into master will decrease coverage by 0.04%.
The diff coverage is n/a.

@@            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%)	⬇️
#single	40.33% <ø> (-0.11%)	⬇️

Impacted Files	Coverage Δ
pandas/io/gbq.py	25% <0%> (-58.34%)	⬇️
pandas/plotting/_converter.py	63.23% <0%> (-1.82%)	⬇️
pandas/core/common.py	90.68% <0%> (-0.35%)	⬇️
pandas/core/frame.py	97.59% <0%> (-0.1%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 81aa70c...962df50. Read the comment docs.

[jreback]

thanks!