TST: Tests and Helpers for Datetime/Period Arrays
Implement ABCDatetimeArray, ABCTimedeltaArray

Fix some problems in DatetimeArray comparison methods, with tests

Implement assert_datetime_array_equal, still need to do the same for TimedeltaArray

Extend fixtures to allow us to run some of the arithmetic tests on PeriodArray, DatetimeArray.  Change a few usages to use these fixtures.

This is a companion-PR to #23493.  Without that one, we can't easily extend the existing tests.  Without this one, problems in the comparison methods would cause failures.

