Use ConcurrentSet in testTrackingChannelTask (#56775)

We need to use a ConcurrentSet to track the canceled tasks 
as cancelTaskAndDescendants can be called concurrently.

Closes #56746

Author: dnhatn

server/src/test/java/org/elasticsearch/tasks/TaskManagerTests.java

@@ -79,7 +79,7 @@ public void testResultsServiceRetryTotalTime() {
     @AwaitsFix(bugUrl = "https://github.com/elastic/elasticsearch/issues/56746")
     public void testTrackingChannelTask() throws Exception {
         final TaskManager taskManager = new TaskManager(Settings.EMPTY, threadPool, Set.of());
-        Set<CancellableTask> cancelledTasks = new HashSet<>();
+        Set<CancellableTask> cancelledTasks = ConcurrentCollections.newConcurrentSet();
         taskManager.setTaskCancellationService(new TaskCancellationService(mock(TransportService.class)) {
             @Override
             void cancelTaskAndDescendants(CancellableTask task, String reason, boolean waitForCompletion, ActionListener<Void> listener) {