Separate TimedeltaIndex mul/div tests
Collect DTI/TDI/PI indexing tests ~symmetrically.
> Collect DTI/TDI/PI indexing tests ~symmetrically.

@jbrockmendel : Can you clarify what you mean by that? thanks. pls parameterize / de-dupe on next passes > Can you clarify what you mean by that?

@gfyoung Each of datetimes/timedeltas/periods have a test_indexing.py file, but they aren't (weren't) consistent about what methods get tested in that file.  This aligned the organization between these three files.