Add expectNextChainingPF method to streams testkit #21614 (#21617)

Add expectNextChainingPF method to streams testkit #21614

Improved error messages on expectNextPf, expectNextPfChaining, expectEventPF

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

@@ -530,14 +530,21 @@ object TestSubscriber {
 
     def expectNextPF[T](f: PartialFunction[Any, T]): T = {
       expectEventPF {
-        case OnNext(n) ⇒
-          assert(f.isDefinedAt(n))
-          f(n)
+        case OnNext(n) if f.isDefinedAt(n) ⇒ f(n)
       }
     }
 
+    /**
+     * Expect next element and test it with partial function.
+     *
+     * Allows chaining probe methods.
+     */
+    def expectNextChainingPF(f: PartialFunction[Any, Any]): Self = {
+      expectNextPF(f.andThen(_ ⇒ self))
+    }
+
     def expectEventPF[T](f: PartialFunction[SubscriberEvent, T]): T =
-      probe.expectMsgPF[T]()(f.asInstanceOf[PartialFunction[Any, T]])
+      probe.expectMsgPF[T](hint = "message matching partial function")(f.asInstanceOf[PartialFunction[Any, T]])
 
     /**
      * Receive messages for a given duration or until one does not match a given partial function.