REF/TST: Collect Straggler Arithmetic Tests
After this AFAICT there are only 5 files left with possibly-misplaced arithmetic tests.  Leaving those for another pass since some of them are testing type-specific behavior so will need a careful look.

Moved tests are unchanged except for contextifying pytest.raises where appropriate.

There's a hackathon in Feb I'm vaguely planning to use as a time to try to de-duplicate/parametrize the tests that have been moved to tests.arithmetic with TODO notes.