Add errors to exercises that compile without user changes

exercises/test2.rs

@@ -17,6 +17,11 @@ mod tests {
 
     #[test]
     fn returns_twice_of_positive_numbers() {
-        assert_eq!(4, 4);
+        assert_eq!(times_two(4), ???);
+    }
+
+    #[test]
+    fn returns_twice_of_negative_numbers() {
+        // TODO write an assert for `times_two(-4)`
     }
 }