Fix TasksIT.testTasksUnblocking (#95584)

* Fix TasksIT.testTasksUnblocking

Due to #94865, we now send an ack before the task gets removed, so we can see
a non-zero amount of tasks in the task manager for a short amount of time.

We can just busy wait until the task gets removed.

See #95494, #94987

server/src/internalClusterTest/java/org/elasticsearch/action/admin/cluster/node/tasks/TasksIT.java

@@ -532,7 +532,12 @@ public void testTasksUnblocking() throws Exception {
         new TestTaskPlugin.UnblockTestTasksRequestBuilder(client(), TestTaskPlugin.UnblockTestTasksAction.INSTANCE).get();
 
         future.get();
-        assertEquals(0, clusterAdmin().prepareListTasks().setActions(TestTaskPlugin.TestTaskAction.NAME + "[n]").get().getTasks().size());
+        assertBusy(
+            () -> assertEquals(
+                0,
+                clusterAdmin().prepareListTasks().setActions(TestTaskPlugin.TestTaskAction.NAME + "[n]").get().getTasks().size()
+            )
+        );
     }
 
     public void testListTasksWaitForCompletion() throws Exception {