TST: Split test_excel into sub test files

[WillAyd]

Logical follow up to https://github.com/pandas-dev/pandas/pull/24423/files#r244284618

cc @gfyoung

[stevenbw]

Hi @WillAyd, is this issue still open? If so I would be interested in giving it a shot :)

[WillAyd]

@stevenbw for sure - PRs are always welcome!

[stevenbw]

@WillAyd Just want to make sure I am interpreting this correctly. The only change is to move the TestXlrdReader class into its own file from test_excel?

[WillAyd]

At the very least want to create a sub directory for excel which has a base.py and an xlrd.py which logically separate the tests. There may be other logical organization points so if you have ideas on that certainly open to it.

Cc @gfyoung if you have any other thoughts to offer

[gfyoung]

@WillAyd : I think you explained it pretty well here!

@stevenbw : Just a point of guidance. The idea is to hopefully create smaller, more modular tests in the spirit of pytest, hence why we would like to split the tests up instead of having one massive file.

There might also be opportunities to leverage more pytest idiom such as fixtures and parameterizations, but we can cross that bridge later if need be.

[stevenbw]

@WillAyd @gfyoung thanks for your help. I have split the file in two as you suggested, just reviewing before I open a PR. There could be an opportunity to separate the openpyxl tests out.

As @gfyoung said, it may be worthwhile undertaking the bigger task of refactoring the test files to a more functional style rather than class based.