Added 2 tests to list.rs, improve semantic and using Default trait instant of function new

src/kernel/thread_manager.rs

@@ -28,8 +28,8 @@ impl ThreadManager {
         Self { 
             g_current_thread: Option::None, 
             g_thread_to_be_destroyed: Option::None, 
-            g_alive: List::new(),
-            ready_list: List::new(),
+            g_alive: List::default(),
+            ready_list: List::default(),
         }
     }