Subcribe can't work in the method with @Test

I have found that the client.subcribe() can not work in the @Test, but client.publish() work.

My code is as follows:
`
String host = "tcp://***.**.**.**:1883";
String clientid = UUID.randomUUID().toString();
MqttClient client = new MqttClient(host, clientid, new MemoryPersistence());

MqttConnectOptions options = new MqttConnectOptions();
options.setCleanSession(true);
options.setConnectionTimeout(10);
options.setKeepAliveInterval(20);
client.setCallback(new MessageCallback());

client.connect(options);

client.subscribe("topic", 0);
`

It can successfully connect but will trigger socker error.
This problem only exits in the @Test.

- [x] Bug exists Release Version 1.2.3 ( Master Branch)
- [ ] Bug exists in MQTTv3 Client on Snapshot Version 1.2.4-SNAPSHOT (Develop Branch)
- [ ] Bug exists in MQTTv5 Client on Snapshot Version 1.2.3-SNAPSHOT (Develop Branch)

If this is a bug regarding the Android Service, please raise the bug here instead: https://github.com/eclipse/paho.mqtt.android/issues/new