TST: do not use int to negative int power in tests (GH14489)
Closes #14489

Numpy starts to error on those cases from 1.12 https://github.com/numpy/numpy/pull/8127

This just fixes the test for now, but other option is to coerce to floats (but that is certainly not for 0.19.1)

hah I think we duped on this one.......
 Yep, I also just see it :-) Yours is cleaner, mine still tests negative ints to positive powers
 hahah, ok.
