Add unit test for check_timelocks API

Add a unit test that attempts to parse a `concrete::Policy` with both
absolute height and time locks.

Done in preparation for patching `check_timelocks_helper`.

Author: tcharding

src/policy/concrete.rs

@@ -1267,4 +1267,11 @@ mod tests {
         let policy = Policy::<String>::from_str("or(and(pk(A),pk(B)),pk(C))").unwrap();
         assert_eq!(policy.num_tap_leaves(), 2);
     }
+
+    #[test]
+    #[should_panic]
+    fn check_timelocks() {
+        // This implicitly tests the check_timelocks API (has height and time locks).
+        let _ = Policy::<String>::from_str("and(after(10),after(500000000))").unwrap();
+    }
 }