Fix SearchableSnapshotsPrewarmingIntegTests (Backport #101203)

Closes #112422

Author: nicktindall

x-pack/plugin/searchable-snapshots/src/internalClusterTest/java/org/elasticsearch/xpack/searchablesnapshots/cache/full/SearchableSnapshotsPrewarmingIntegTests.java

@@ -272,11 +272,13 @@ public void testConcurrentPrewarming() throws Exception {
         final CountDownLatch startPrewarmingLatch = new CountDownLatch(1);
         final ThreadPool threadPool = getInstanceFromNode(ThreadPool.class);
         final int maxUploadTasks = threadPool.info(CACHE_PREWARMING_THREAD_POOL_NAME).getMax();
+        final CountDownLatch maxUploadTasksCreated = new CountDownLatch(maxUploadTasks);
         for (int i = 0; i < maxUploadTasks; i++) {
             threadPool.executor(CACHE_PREWARMING_THREAD_POOL_NAME).execute(new AbstractRunnable() {
 
                 @Override
                 protected void doRun() throws Exception {
+                    maxUploadTasksCreated.countDown();
                     startPrewarmingLatch.await();
                 }
 
@@ -287,6 +289,7 @@ public void onFailure(Exception e) {
             });
         }
 
+        safeAwait(maxUploadTasksCreated);
         ExecutorService prewarmingExecutor = threadPool.executor(SearchableSnapshots.CACHE_PREWARMING_THREAD_POOL_NAME);
         assertThat(prewarmingExecutor, instanceOf(ThreadPoolExecutor.class));
         assertThat(((ThreadPoolExecutor) prewarmingExecutor).getActiveCount(), equalTo(maxUploadTasks));