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/scaladsl/Source.scala

@@ -324,6 +324,11 @@ object Source extends SourceApply {
    */
   def single[T](element: T): Source[T, Unit] = apply(SynchronousIterablePublisher(List(element), "single")) // FIXME optimize
 
+  /**
+   * Create a `Source` that will continually emit the given element.
+   */
+  def repeat[T](element: T): Source[T, Unit] = apply(() ⇒ Iterator.continually(element)) // FIXME optimize
+
   /**
    * A `Source` with no elements, i.e. an empty stream that is completed immediately for every connected `Sink`.
    */