Add unit tests for most of util.cpp

luprex/core/cpp/world.cpp

@@ -520,7 +520,7 @@ void World::difference_transmit(int64_t actor_id, const World *master, StreamBuf
 
     // Get the list of tangibles visible in either model.
     // Some tangibles may be missing in the master, some may be missing in the sync.
-    util::IdVector visible = PlaneMap::sort_union_id_vectors(
+    util::IdVector visible = util::sort_union_id_vectors(
             master->get_near(actor_id, 100.0, true),
             this->get_near(actor_id, 100.0, true));
     TanVector m_visible = tangible_get_all(visible);