tests to verify correctness of sampling

[YeungOnion]

We could implement a few test statistics for goodness of fit for these distributions, would at least help me for verifying #287

• $\chi^2$, added in Add some hypothesis test functions #315
• Komolgorov-Smirnov
• Anderson-Darling
• Cramér-von Mises

and then we'd also have some features

[DanielLacina]

I'll look into it.