test: try different delay

Author: Vinzent03

packages/realtime_client/test/mock_test.dart

@@ -287,7 +287,7 @@ void main() {
 
       channel.subscribe(subscribeCallback);
 
-      await Future.delayed(Duration(milliseconds: 200));
+      await Future.delayed(Duration(milliseconds: 500));
       await client.conn!.sink
           .close(Constants.wsCloseNormal, "heartbeat timeout");
     });