Collect datetime64 and PeriodDtype arithmetic tests
As suggested, these are split by dtype in a new tests/arithmetic directory.

Started parametrizing a few of these over `box` classes, but for the most part this is just collecting tests from their existing locations.  There is going to be a lot of parametrization and de-duplication coming up.

Holding off on moving the existing timedelta64-dtype tests until #22163 is handled to avoid merge conflict.