Add a test of `utils::factorial`

[JSorngard]

This PR adds a test of the factorial function in puruspe, by calculating the factorial naively in a loop and comparing with the output. I use approximate comparisons because there are different floating point errors at very large outputs.

This adds tests for one of the utils functions for #12.

[JSorngard]

It is possible to test to larger input values, but the exponents on the outputs are so large that the small differences in the mantissa due to different floating point errors between the function and the test code result in values that differ enough to need custom epsilon values.

[JSorngard]

I added a big test case manually