Add a function for extracting a test statistic

[ararslan]

This is added to StatsAPI rather than to HypothesisTests since StatsAPI also houses HypothesisTest, pvalue, and other relevant functionality.

Defining this function will bring a resolution to the 7-year-old issue https://github.com/JuliaStats/HypothesisTests.jl/issues/79, which has received a number of duplicates over the years, suggesting that it would be of general interest.

I considered the name statistic, and still rather prefer that (teststatistic has too many s's and t's 😩), but wasn't sure whether it was insufficiently descriptive. I'd be interested in hearing thoughts both on naming and on whether this should be required for HypothesisTest as pvalue is or whether it should be optional (what I have currently).