MQTT: BackToBackAdapterTests: use TemporaryFolder

https://build.spring.io/browse/INT-MJATS41-38

Use `TemporaryFolder` for `BackToBackAdapterTests#testAsyncPersisted()`
to avoid concurrent builds cross-talk.

There is no need to do anything around the Paho `FileLock`, because the code looks like in the `MqttAsyncClient`:
```
this.persistence.open(clientId, serverURI);
this.comms = new ClientComms(this, this.persistence);
this.persistence.close();
```
So, the lock is released before the real connect to the Broker.