Add extra logging to PrioritizedThrottledTaskRunnerTests

This should help debugging elastic#92910 and rule out possible enviromental
issues, such us slowness creating threads due to an overloaded CI
worker.

server/src/test/java/org/elasticsearch/common/util/concurrent/PrioritizedThrottledTaskRunnerTests.java

@@ -231,9 +231,14 @@ private int getRandomPriority() {
     }
 
     private void assertNoRunningTasks(PrioritizedThrottledTaskRunner<TestTask> taskRunner) {
+        logger.info("--> ensure that there are no running tasks in the executor");
         final var barrier = new CyclicBarrier(maxThreads + 1);
         for (int i = 0; i < maxThreads; i++) {
-            executor.execute(() -> awaitBarrier(barrier));
+            executor.execute(() -> {
+                logger.info("--> await until barrier is released");
+                awaitBarrier(barrier);
+                logger.info("--> the barrier is released");
+            });
         }
         awaitBarrier(barrier);
         assertThat(taskRunner.runningTasks(), equalTo(0));