mathext: fix test
Created by: kortschak
This makes the tests abs or rel, and fixes an expectation that does not match wolfram-alpha, -1.1111...e9 should give cmplx.Inf(), which math.NaN() is close enough to. I don't know why the 30.49... was there in the tests, but it was only passing because got was NaN.
Please take a look.
Closes #306.