Wait for demaind before signalling onNext

Fixes reactive-streams#277.

Fixes two whitebox subscriber tests where they weren't waiting for
demand before signalling onNext.

Author: jroper

Diff for: tck/src/main/java/org/reactivestreams/tck/SubscriberWhiteboxVerification.java

@@ -267,6 +267,7 @@ public void required_spec208_mustBePreparedToReceiveOnNextSignalsAfterHavingCall
       public void run(WhiteboxTestStage stage) throws InterruptedException {
         stage.puppet().triggerRequest(1);
         stage.puppet().signalCancel();
+        stage.expectRequest();
         stage.signalNext();
 
         stage.puppet().triggerRequest(1);
@@ -437,7 +438,12 @@ public void required_spec308_requestMustRegisterGivenNumberElementsToBeProduced(
       @Override
       public void run(WhiteboxTestStage stage) throws InterruptedException {
         stage.puppet().triggerRequest(2);
+        long requestedElements = stage.expectRequest();
         stage.probe.expectNext(stage.signalNext());
+        // Some subscribers may only request one element at a time.
+        if (requestedElements < 2) {
+          stage.expectRequest();
+        }
         stage.probe.expectNext(stage.signalNext());
 
         stage.probe.expectNone();