[CHECK] Add some more factorization exams.

Add the 15 multivariate polynomials from P. S. Wang's paper "An
Improved Multivariate Polynomial Factoring Algorithm" as exams.

Don't add them as benchmarks since timings vary considerably
depending on internal choice of variables. In any case, this should
never take hours. (But before 630db9a0b0 it did, occasionally.)