Add a function for extracting a test statistic

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
JuliaStats/HypothesisTests.jl#79, which has
received a number of duplicates over the years, suggesting that it would
be of general interest.

src/StatsAPI.jl

@@ -37,8 +37,8 @@ function pairwise! end
 
 Abstract supertype for all statistical hypothesis tests.
 Subtypes must implement [`pvalue`](@ref) at a minimum and may also
-implement functions such as [`confint`](@ref), [`nobs`](@ref), and
-[`dof`](@ref) as appropriate.
+implement functions such as [`teststatistic`](@ref), [`confint`](@ref),
+[`nobs`](@ref), and [`dof`](@ref) as appropriate.
 """
 abstract type HypothesisTest end
 
@@ -49,4 +49,11 @@ Compute the p-value for a given significance test.
 """
 function pvalue end
 
+"""
+    teststatistic(test)
+
+Return the test statistic for a given hypothesis test.
+"""
+function teststatistic end
+
 end # module