Fixing test3 to have enough tests to make sure we test all cases

Author: calvinbrown085

exercises/tests/tests3.rs

@@ -18,4 +18,9 @@ mod tests {
     fn is_true_when_even() {
         assert!();
     }
+
+    #[test]
+    fn is_false_when_even() {
+        assert!();
+    }
 }