NO-JIRA fix intermittent failure, this test failed on local repeat getting a null from immediate

tests/integration-tests/src/test/java/org/apache/activemq/artemis/tests/integration/client/SlowConsumerTest.java

@@ -210,6 +210,9 @@ public class SlowConsumerTest extends ActiveMQTestBase {
       ClientConsumer consumer = addClientConsumer(session.createConsumer(QUEUE));
       session.start();
 
+      Queue queue = server.locateQueue(QUEUE);
+      Wait.assertTrue(() -> queue.getDeliveringCount() >= 3 * threshold);
+
       for (int i = 0; i < 3 * threshold; i++) {
          consumer.receiveImmediate().individualAcknowledge();
       }