Update dtype strategies for elementwise/operators tests #60

Many of the elementwise/operator tests were written a while back, and there has been a lot of updates since in regards to what dtypes are allowed. This means we currently don't test all possible inputs!

Bug found today by @IvanYashchuk was numpy.array_api.pow() not working for integer arrays, probably because of the test suite was not updated with the spec.

So I'll need to review all the test methods to identify which aren't using all valid dtypes, and subsequently update them.