Use global rand.Rand instance in mathrand.

common/data/rand/mathrand/mathrand_test.go

Index: common/data/rand/mathrand/mathrand_test.go
diff --git a/common/data/rand/mathrand/mathrand_test.go b/common/data/rand/mathrand/mathrand_test.go
index 7138be652d21197f50057a77bdc0edebb97e8923..72bb8b468998ceab1b55d956cfee58cb8913fcb7 100644
--- a/common/data/rand/mathrand/mathrand_test.go
+++ b/common/data/rand/mathrand/mathrand_test.go
@@ -22,6 +22,10 @@ func Test(t *testing.T) {
 		Convey("unset", func() {
 			// Just ensure doesn't crash.
 			So(Get(c).Int()+1 > 0, ShouldBeTrue)
+			So(WithGoRand(c, func(r *rand.Rand) error {
+				So(r.Int(), ShouldBeGreaterThanOrEqualTo, 0)
+				return nil
+			}), ShouldBeNil)
 		})
 
 		Convey("set persistance", func() {