fix EventStreamSpec instability

fixes #30460

akka-actor-tests/src/test/scala/akka/event/EventStreamSpec.scala

@@ -4,14 +4,18 @@
 
 package akka.event
 
+import akka.Done
+
 import scala.concurrent.duration._
-
 import com.typesafe.config.ConfigFactory
-import language.postfixOps
 
+import language.postfixOps
 import akka.actor._
 import akka.testkit.{ AkkaSpec, GHExcludeTest, TestProbe }
 
+import scala.concurrent.{ ExecutionContext, Promise }
+import scala.util.Try
+
 object EventStreamSpec {
 
   val config = ConfigFactory.parseString("""
@@ -71,6 +75,7 @@ class EventStreamSpec extends AkkaSpec(EventStreamSpec.config) {
   import EventStreamSpec._
 
   val impl = system.asInstanceOf[ActorSystemImpl]
+  implicit val ec: ExecutionContext = system.dispatcher
 
   "An EventStream" must {
 
@@ -337,13 +342,26 @@ class EventStreamSpec extends AkkaSpec(EventStreamSpec.config) {
         es.subscribe(target, classOf[A]) should ===(true)
         fishForDebugMessage(a2, s"unsubscribing $target from all channels")
 
-        es.subscribe(target, classOf[A]) should ===(true)
-        fishForDebugMessage(a2, s"unsubscribing $target from all channels")
+        after(30.millis) {
+          es.subscribe(target, classOf[A]) should ===(true)
+          fishForDebugMessage(a2, s"unsubscribing $target from all channels")
+        }
       } finally {
         shutdown(sys)
       }
     }
 
+    def after(delay: FiniteDuration)(block: => Unit): Unit = {
+      val done = Promise[Done]()
+      system.scheduler.scheduleOnce(delay) {
+        done.complete(Try {
+          block
+          Done
+        })
+      }
+      done.future.futureValue
+    }
+
     "not allow initializing a TerminatedUnsubscriber twice" in {
       val sys = ActorSystem("MustNotAllowDoubleInitOfTerminatedUnsubscriber", config)
       // initializes an TerminatedUnsubscriber during start