add a NotOk helper method in the testing package

util/testutil/testing.go

@@ -48,6 +48,15 @@ func Ok(tb testing.TB, err error) {
 	}
 }
 
+// NotOk fails the test if an err is nil.
+func NotOk(tb testing.TB, err error) {
+	if err == nil {
+		_, file, line, _ := runtime.Caller(1)
+		fmt.Printf("\033[31m%s:%d: expected error, got nothing \033[39m\n\n", filepath.Base(file), line)
+		tb.FailNow()
+	}
+}
+
 // Equals fails the test if exp is not equal to act.
 func Equals(tb testing.TB, exp, act interface{}) {
 	if !reflect.DeepEqual(exp, act) {