Excel Test Cleanup - ReadWriteClass

[WillAyd]

The tests we have in this module need a pretty big refactor as they mix a few testing idioms together. This is slowing down contributions on items like #25092 and #25427

This is going to require quite a few PRs to get it where it needs to be. This one simply:

• Moves tests that require both reading and writing into a dedicated class
• Moves skip fixtures off of the SharedItems class onto the classes that actually use them

I'd eventually like to get rid of the SharedItems class and use parametrization on Reader/Writer classes to test the various combinations of engines and extensions, though again going to take a few PRs to get there

[codecov]

Codecov Report

Merging #26473 into master will decrease coverage by 50.05%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master   #26473       +/-   ##
===========================================
- Coverage   91.75%    41.7%   -50.06%     
===========================================
  Files         174      174               
  Lines       50765    50765               
===========================================
- Hits        46578    21170    -25408     
- Misses       4187    29595    +25408

Flag	Coverage Δ
#multiple	?
#single	41.7% <ø> (-0.11%)	⬇️

Impacted Files	Coverage Δ
pandas/io/formats/latex.py	0% <0%> (-100%)	⬇️
pandas/io/sas/sas_constants.py	0% <0%> (-100%)	⬇️
pandas/core/groupby/categorical.py	0% <0%> (-100%)	⬇️
pandas/tseries/plotting.py	0% <0%> (-100%)	⬇️
pandas/tseries/converter.py	0% <0%> (-100%)	⬇️
pandas/io/formats/html.py	0% <0%> (-99.37%)	⬇️
pandas/io/sas/sas7bdat.py	0% <0%> (-91.16%)	⬇️
pandas/io/sas/sas_xport.py	0% <0%> (-90.1%)	⬇️
pandas/core/tools/numeric.py	10.14% <0%> (-89.86%)	⬇️
pandas/io/s3.py	0% <0%> (-89.48%)	⬇️
... and 129 more

---

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 f5cc078...5c30f32. Read the comment docs.

[codecov]

Codecov Report

Merging #26473 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #26473      +/-   ##
==========================================
- Coverage   91.75%   91.74%   -0.01%     
==========================================
  Files         174      174              
  Lines       50765    50765              
==========================================
- Hits        46578    46574       -4     
- Misses       4187     4191       +4

Flag	Coverage Δ
#multiple	90.25% <ø> (ø)	⬆️
#single	41.72% <ø> (-0.09%)	⬇️

Impacted Files	Coverage Δ
pandas/io/gbq.py	78.94% <0%> (-10.53%)	⬇️
pandas/core/frame.py	97.02% <0%> (-0.12%)	⬇️

---

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 f5cc078...5c30f32. Read the comment docs.

[WillAyd]

It might not be entirely apparent from the diff but to clarify here I've moved get_csv_refdf, get_excelfile and get_exceldf out of the SharedItems class because they were only used by the ReaderBaseClass.

What's left in SharedItems should be replaced by fixtures where needed in the next PR

[jreback]

thanks @WillAyd cleanups for testing here very welcome!