Enable Authorization Testkit tests for reactive backend (neo4j#1063)

This update also makes sure that `DemandConsumer` completes immediately when there is no unconsumed demand.

Author: injectives

testkit-backend/src/main/java/neo4j/org/testkit/backend/messages/requests/SessionClose.java

@@ -129,7 +129,12 @@ private DemandConsumer( RxBlockingSubscriber<T> subscriber, AtomicLong unfulfill
             this.unfulfilledDemandCounter = unfulfilledDemandCounter;
 
             subscriber.getCompletionStage().whenComplete( this::onComplete );
-            if ( this.unfulfilledDemandCounter.get() > 0 )
+            long unfulfilledDemand = this.unfulfilledDemandCounter.get();
+            if ( unfulfilledDemand == 0 )
+            {
+                completedStage.complete( CompletionReason.REQUESTED_DEMAND_CONSUMED );
+            }
+            else if ( unfulfilledDemand > 0 )
             {
                 setupNextSignalConsumer();
             }