Flaky test fix for StoringSubscriberTest (#6420)

* flaky test fix

* changelog removed

Author: Fred1155

utils/src/test/java/software/amazon/awssdk/utils/async/StoringSubscriberTest.java

@@ -159,8 +159,6 @@ public void cancel() {
                 }
             };
 
-            subscriber.onSubscribe(subscription);
-
             Future<Object> consumerFuture = consumer.submit(() -> {
                 int expectedMessageNumber = 0;
                 while (testRunning.get()) {
@@ -183,6 +181,8 @@ public void cancel() {
                 return null;
             });
 
+            subscriber.onSubscribe(subscription);
+
             Thread.sleep(5_000);
             testRunning.set(false);
             consumerFuture.get();