Mann-Whitney U test should return p-value.

I noticed that while it is possible to get the p-value from some tests, many do not have this option. For instance, `mannWhitneyUtest`. One issue I believe is the `TestResult` type which over simplifies the result of a test. Instead of a binary type, this type should contain the p-value, critical values, and maybe even sample sizes, but definitely a p-value.