metal: allow changing the connector mode

src/it/test_backend.rs

@@ -246,6 +246,10 @@ impl Connector for TestConnector {
     fn set_enabled(&self, _enabled: bool) {
         // todo
     }
+
+    fn set_mode(&self, _mode: Mode) {
+        // todo
+    }
 }
 
 pub struct TestMouseClick {