Excel Tests Continued Fixture Cleanup
Follow up to #26579

Removing the frame2 fixture which is sparsely used. I've also removed inheritance of `ReadingTestsBase` and made that it's own discoverable class. Previously these tests were only being generated when `TestXlrdReader` was being run which tightly couples reading with xlrd. 

The goal here in subsequent PRs would be to better parametrize `TestReaders` for new engines that come in and truly make `TestXlrdReader` only deal with xlrd specific tests (will have to move around some tests to make that happen)
