Generate random polys whose prs is not normal

[wbhart]

Given polynomials f and g, their polynomial remainder sequence is said to be normal if all the differences of degree are 1. For polynomials f and g whose prs is not normal, different paths are hit in the resultant and gcd algorithms. However, hitting such polynomials randomly is very hard. We should add a rand_nonnormal which constructs a pair of such polynomials (by reversing the prs algorithm). This should then be used to test all the gcd and resultant functionality.