En gros ça sert a dire a akka qu'il n'attend pas de message: "After that you can use expectMsg to assert that a specific message has been received, or expectNoMsg to assert that no message should have been received" de (http://roestenburg.agilesquad.com/2011/02/unit-testing-akka-actors-with-testkit_12.html)
En gros si je mets pas cette ligne le test fail car akka attend un message.