#22463 Fix race conditions in FlowScan(Async)Spec (#22474)

between error propagation and first element.
Such race condition surfaced after materialization
time decreased substantially.

akka-stream-tests/src/test/scala/akka/stream/scaladsl/FlowScanAsyncSpec.scala

@@ -30,8 +30,7 @@ class FlowScanAsyncSpec extends StreamSpec {
         .via(sumScanFlow)
         .runWith(TestSink.probe[Int])
         .request(1)
-        .expectNext(0)
-        .expectComplete()
+        .expectNextOrComplete(0)
     }
 
     "work with a single source" in {
@@ -73,7 +72,8 @@ class FlowScanAsyncSpec extends StreamSpec {
       Source.failed[Int](expected)
         .via(sumScanFlow)
         .runWith(TestSink.probe[Int])
-        .expectSubscriptionAndError(expected)
+        .request(2)
+        .expectNextOrError(0, expected)
     }
 
     "with the restarting decider" should {

akka-stream-tests/src/test/scala/akka/stream/scaladsl/FlowScanSpec.scala

@@ -71,10 +71,14 @@ class FlowScanSpec extends StreamSpec {
         .expectNext(0)
         .expectComplete()
     }
+
     "fail when upstream failed" in {
-      Source.failed[Int](TE("")).scan(0) { case (a, b) ⇒ a + b }.runWith(TestSink.probe[Int])
+      val ex = TE("")
+      Source.failed[Int](ex)
+        .scan(0) { case (a, b) ⇒ a + b }
+        .runWith(TestSink.probe[Int])
         .request(2)
-        .expectError(TE(""))
+        .expectNextOrError(0, ex)
     }
   }
 }