Use assertBusy for task unregistration events (#96347)

I think this also relates to reversing the order of sending response and
unregistration in #94865.

Closes #96103

Author: pxsalehi

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

@@ -132,20 +132,24 @@ public void testTaskCounts() {
         assertThat(response.getTasks().size(), greaterThanOrEqualTo(cluster().numDataNodes()));
     }
 
-    public void testMasterNodeOperationTasks() {
+    public void testMasterNodeOperationTasks() throws Exception {
         registerTaskManagerListeners(ClusterHealthAction.NAME);
 
         // First run the health on the master node - should produce only one task on the master node
         internalCluster().masterClient().admin().cluster().prepareHealth().get();
         assertEquals(1, numberOfEvents(ClusterHealthAction.NAME, Tuple::v1)); // counting only registration events
-        assertEquals(1, numberOfEvents(ClusterHealthAction.NAME, event -> event.v1() == false)); // counting only unregistration events
+        // counting only unregistration events
+        // When checking unregistration events there might be some delay since receiving the response from the cluster doesn't
+        // guarantee that the task has been unregistered.
+        assertBusy(() -> assertEquals(1, numberOfEvents(ClusterHealthAction.NAME, event -> event.v1() == false)));
 
         resetTaskManagerListeners(ClusterHealthAction.NAME);
 
         // Now run the health on a non-master node - should produce one task on master and one task on another node
         internalCluster().nonMasterClient().admin().cluster().prepareHealth().get();
         assertEquals(2, numberOfEvents(ClusterHealthAction.NAME, Tuple::v1)); // counting only registration events
-        assertEquals(2, numberOfEvents(ClusterHealthAction.NAME, event -> event.v1() == false)); // counting only unregistration events
+        // counting only unregistration events
+        assertBusy(() -> assertEquals(2, numberOfEvents(ClusterHealthAction.NAME, event -> event.v1() == false)));
         List<TaskInfo> tasks = findEvents(ClusterHealthAction.NAME, Tuple::v1);
 
         // Verify that one of these tasks is a parent of another task