fix two wrong/flaky tests

- ordering is not preserved by shufflers in GraphOpsIntegrationSpec
- larger tolerance is needed in GraphBalanceSpec since balancing does
  not keep track of previous imbalances
- also add Source.repeat(elem)

akka-stream/src/main/scala/akka/stream/javadsl/Source.scala

@@ -134,6 +134,12 @@ object Source {
   def single[T](element: T): Source[T, Unit] =
     new Source(scaladsl.Source.single(element))
 
+  /**
+   * Create a `Source` that will continually emit the given element.
+   */
+  def repeat[T](element: T): Source[T, Unit] =
+    new Source(scaladsl.Source.repeat(element))
+
   /**
    * Create a `Source` that immediately ends the stream with the `cause` failure to every connected `Sink`.
    */