[POC] implement test_arithmetic.py
There are a ton of scattered arithmetic tests for Index/Series/DataFrame that _should_ be testing the same things, but in fact are haphazard.  Fixing this given the current structure would entail an enormous about of code duplication.

This PR if a proof of concept for gathering all those tests in one test_arithmetic.py file, parametrizing them, and ensuring that the relevant behavior is identical across arraylike classes.

As Datetime/Timedelta/Period EA come online, the case for de-duplication will be even stronger.

In this form it is really easy to track (via xfails) what behavior needs fixing.