title: How to disable Akka Dead Letters warnings in Tests
body: <p>I recently upgraded to Akka 2.6.10. Now I'm seeing a lot of warnings like this when I run tests for my actors with <code>ActorTestKit</code>:</p>
  <pre><code>[WARN] [12/16/2020 09:03:14.995] [AsyncTestWithControlledTime-akka.actor.internal-dispatcher-3] [akka://AsyncTestWithControlledTime/user/$a] received dead letter: GotCriticalFailure(my custom error message,akka.stream.AbruptStageTerminationException: GraphStage [akka.stream.impl.ActorRefSinkStage$$anon$1@15cc9eaa] terminated abruptly, caused by for example materializer or actor system termination.)
  </code></pre>
  <p>AyncTestWithControlledTime is a class I wrote and is a super class of all my tests. It creates the test kit and provides the config for the actor sytem. However, trying to suppress the warnings via the config, for instance with <code>akka.log-dead-letters = off</code>, does not help.</p>
  <p>I usually receive the warnings when my actor has subscribed to a stream and the test finishes so the stream is not properly cancelled.</p>
  <p>How can I disable the warnings. It would be best if I could suppress them only when the test is finished and the test kit actor system is shut down.</p>