subscribe: set client_id for MQTT subscriber to ensure unique identification

test/subscribe.py

@@ -32,6 +32,7 @@ client.on_connect = on_connect
 client.on_message = on_message
 if username:
     client.username_pw_set(username, password)
+    client.client_id = f"subscribe-{os.getpid()}"
 
 try:
     client.connect(host, port, 60)