add a 'REQUIRE' for the moon integration test

tests/test_moon_orbits.cpp

@@ -260,5 +260,7 @@ TEST_CASE("Combined solar system with all moons - parent stability", "[moon][int
              << " at day " << change.time_days);
     }
 
+    REQUIRE(parent_changes.size() == 0);
+
     destroy_simulation(sim);
 }