MqttSessionSpec: MQTT server connector should unsubscribe a server session

[ennru]

In #2433

[info] MqttSessionSpec:
[info] MQTT server connector
[info] - should unsubscribe a server session *** FAILED *** (3 seconds, 170 milliseconds)
java.lang.AssertionError: assertion failed: timeout (3 seconds) during expectMsg while waiting for ByteString(50, 26, 0, 10, 115, 111, 109, 101, 45, 116, 111, 112, 105, 99, 0, 1, 115, 111, 109, 101, 45, 112, 97, 121, 108, 111, 97, 100)
[info]   at scala.Predef$.assert(Predef.scala:223)
[info]   at akka.testkit.TestKitBase.expectMsg_internal(TestKit.scala:459)
[info]   at akka.testkit.TestKitBase.expectMsg(TestKit.scala:436)
[info]   at akka.testkit.TestKitBase.expectMsg$(TestKit.scala:436)
[info]   at akka.testkit.TestKit.expectMsg(TestKit.scala:969)
[info]   at docs.scaladsl.MqttSessionSpec.$anonfun$new$120(MqttSessionSpec.scala:1546)

https://travis-ci.com/github/akka/alpakka/jobs/395719263#L787

Not sure if related, but this caught my eye in the full logs:

2020-10-06 07:08:36,758 ERROR [RequestStateSpec-akka.actor.default-dispatcher-6] [akka.actor.SupervisorStrategy]  Watch(Actor[akka://RequestStateSpec/user/$c#1953167027], Actor[akka://RequestStateSpec/system/testProbe-124#1246072292]) termination message was not overwritten from [Some(Unregister(PacketId(1)))] to [Some(Unregister(PacketId(2)))]. If this was intended, unwatch first before using `watch` / `watchWith` with another message.

java.lang.IllegalStateException: Watch(Actor[akka://RequestStateSpec/user/$c#1953167027], Actor[akka://RequestStateSpec/system/testProbe-124#1246072292]) termination message was not overwritten from [Some(Unregister(PacketId(1)))] to [Some(Unregister(PacketId(2)))]. If this was intended, unwatch first before using `watch` / `watchWith` with another message.
	at akka.actor.dungeon.DeathWatch.checkWatchingSame(DeathWatch.scala:112)
	at akka.actor.dungeon.DeathWatch.watchWith(DeathWatch.scala:46)
	at akka.actor.dungeon.DeathWatch.watchWith$(DeathWatch.scala:38)
	at akka.actor.ActorCell.watchWith(ActorCell.scala:410)
	at akka.actor.typed.internal.adapter.ActorContextAdapter.watchWith(ActorContextAdapter.scala:114)
	at akka.stream.alpakka.mqtt.streaming.impl.LocalPacketRouter.$anonfun$main$1(RequestState.scala:436)

https://travis-ci.com/github/akka/alpakka/jobs/395719263#L1589