Fix stats feature on 1.9 and later

In v1.9, Base.cumulative_compile_time_ns() was changed to return a tuple of
`(compile_time, recompile_time)`. We now use the sum of the two to show the
total compilation time.

src/testset.jl

@@ -672,9 +672,12 @@ macro stats(yes, ex)
     end
 end
 
-cumulative_compile_time_ns() =
-    isdefined(Base, :cumulative_compile_time_ns) ?
+@static if VERSION > v"1.9"
+    cumulative_compile_time_ns() = sum(Base.cumulative_compile_time_ns())
+else
+    cumulative_compile_time_ns() = isdefined(Base, :cumulative_compile_time_ns) ?
         Base.cumulative_compile_time_ns() :
         UInt(0)
+end
 
 end # module