Fix CancellableRateLimitedFluxIteratorTests#testCancellation (#87641)

Ensure the test cancellation consumer provides enough visibility.

Closes #87112

modules/repository-azure/src/test/java/org/elasticsearch/repositories/azure/CancellableRateLimitedFluxIteratorTests.java

@@ -10,6 +10,7 @@ package org.elasticsearch.repositories.azure;
 
 import reactor.core.publisher.Flux;
 
+import org.elasticsearch.common.util.concurrent.ConcurrentCollections;
 import org.elasticsearch.test.ESTestCase;
 import org.elasticsearch.threadpool.TestThreadPool;
 import org.elasticsearch.threadpool.ThreadPool;
@@ -148,10 +149,8 @@ public class CancellableRateLimitedFluxIteratorTests extends ESTestCase {
         iterator.cancel();
     }
 
-    @AwaitsFix(bugUrl = "https://github.com/elastic/elasticsearch/issues/87112")
     public void testCancellation() throws Exception {
         int requestedElements = 4;
-        final AtomicBoolean cancelled = new AtomicBoolean();
         Publisher<Integer> publisher = s -> runOnNewThread(() -> {
             s.onSubscribe(new Subscription() {
                 final CountDownLatch cancellationLatch = new CountDownLatch(1);
@@ -177,13 +176,12 @@ public class CancellableRateLimitedFluxIteratorTests extends ESTestCase {
 
                 @Override
                 public void cancel() {
-                    cancelled.set(true);
                     cancellationLatch.countDown();
                 }
             });
         });
 
-        Set<Integer> cleanedElements = new HashSet<>();
+        final Set<Integer> cleanedElements = ConcurrentCollections.newConcurrentSet();
         CancellableRateLimitedFluxIterator<Integer> iterator = new CancellableRateLimitedFluxIterator<>(
             requestedElements,
             cleanedElements::add