Add unit test that checks exponentiation results for each bignum

Haven't set up pipeline, because that requires a) having runners with an nvidia card and b) having a docker image with the CUDA toolkit (probably), which is freaking huge. The test passes on my machine and I've verified that there are different results for each exponentiated number. Setting the pipeline up to test gpumaths is gonna be another whole thing, so I recommend we wait and hope it never becomes a problem.