Fix test - wait for other threads before throwing the exception (#124386) (#124427)

(cherry picked from commit b9a9784)

# Conflicts:
#	server/src/internalClusterTest/java/org/elasticsearch/search/SearchCancellationIT.java

Author: smalyshev

server/src/internalClusterTest/java/org/elasticsearch/search/SearchCancellationIT.java

@@ -262,12 +262,16 @@ public void testCancelFailedSearchWhenPartialResultDisallowed() throws Exception
         // When the search request executes, block all shards except 1.
         final List<SearchShardBlockingPlugin> searchShardBlockingPlugins = initSearchShardBlockingPlugin();
         AtomicBoolean letOneShardProceed = new AtomicBoolean();
+        // Ensure we have at least one task waiting on the latch
+        CountDownLatch waitingTaskLatch = new CountDownLatch(1);
         CountDownLatch shardTaskLatch = new CountDownLatch(1);
         for (SearchShardBlockingPlugin plugin : searchShardBlockingPlugins) {
             plugin.setRunOnNewReaderContext((ReaderContext c) -> {
                 if (letOneShardProceed.compareAndSet(false, true)) {
                     // Let one shard continue.
                 } else {
+                    // Signal that we have a task waiting on the latch
+                    waitingTaskLatch.countDown();
                     safeAwait(shardTaskLatch); // Block the other shards.
                 }
             });
@@ -280,6 +284,9 @@ public void testCancelFailedSearchWhenPartialResultDisallowed() throws Exception
             plugin.disableBlock();
             plugin.setBeforeExecution(() -> {
                 if (oneThreadWillError.compareAndSet(false, true)) {
+                    // wait for some task to get to the latch
+                    safeAwait(waitingTaskLatch);
+                    // then throw the exception
                     throw new IllegalStateException("This will cancel the ContextIndexSearcher.search task");
                 }
             });