TST: verify we can add and subtract from indices (#8142)
This just provides a validating test that some basic arithmetic on int64 and float64 indices works, and in particular that things like `pd.Int64Index([1, 2]) - 1` no longer raise `Exception: Input must be iterable!`.  

Should be able to close #8142, which in practice hasn't been applicable for a long time.
thanks!