fix(mqtt-sn): fix mqtt-sn test cases

apps/emqx_sn/test/emqx_sn_protocol_SUITE.erl

@@ -1278,6 +1278,7 @@ t_asleep_test06_to_awake_qos2_dl_msg(_) ->
     CleanSession = 0,
     ReturnCode = 0,
     send_register_msg(Socket, TopicName_tom, MsgId1),
+    timer:sleep(50),
     TopicId_tom = check_regack_msg_on_udp(MsgId1, receive_response(Socket)),
     send_subscribe_msg_predefined_topic(Socket, QoS, TopicId_tom, MsgId1),
     ?assertEqual(<<8, ?SN_SUBACK, Dup:1, QoS:2, Retain:1, WillBit:1, CleanSession:1,