REF: refactor cumulative op tests from test_analytics
The removal of `ix` highlighted how difficult it is to tell how thorough tests are for some funcs/methods.  This is one of several PRs intended to clarify where the tests for X live, and ideally to make that follow a standard pattern across DataFrame/Series/Index subclasses (and EA where relevant)
thanks. i kind of like the classes as well, useful for organizing, but equally can just make a directory and a no-class structure. prob not worth the churn though.