+str #18556 add delay combinator

akka-stream-testkit/src/main/scala/akka/stream/testkit/StreamTestKit.scala

@@ -263,6 +263,16 @@ object TestSubscriber {
       self
     }
 
+    /**
+     * Fluent DSL
+     *
+     * Expect a stream element during specified time, then timeout.
+     */
+    def expectNext(d: FiniteDuration, element: I): Self = {
+      probe.expectMsg(d, OnNext(element))
+      self
+    }
+
     /**
      * Fluent DSL
      *