Excel Test Cleanup - ReadWriteClass
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