Create tests for arithmetic for `TimedeltaIndex` and `DatetimeIndex`

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For Series[Timedelta] and Series[Timestamp], comprehensive tests for arithmetic have been added and passed. We would like to have a parallel set of tests for TimedeltaIndex and DatetimeIndex.

Originally posted by @Dr-Irv in #1273 (review)

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Optionally, we can also do the following: in class Timedelta and class Timestamp, move operations related to DatetimeIndex and TimedeltaIndex to the latter two classes, so that Timedelta and Timestamp know as little as possible about Indexes.

Originally posted by @Dr-Irv in #1273 (comment)