Fix TasksIT#testTasksCancellation (#109929)

The tasks are removed from the task manager _after_ sending the
response, so we cannot reliably assert they're done. With this commit we
wait for them to complete properly first.

Closes #109686

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

@@ -488,7 +488,6 @@ public void onTaskRegistered(Task task) {
         }
     }
 
-    @AwaitsFix(bugUrl = "https://github.com/elastic/elasticsearch/issues/109686")
     public void testTasksCancellation() throws Exception {
         // Start blocking test task
         // Get real client (the plugin is not registered on transport nodes)
@@ -511,6 +510,9 @@ public void testTasksCancellation() throws Exception {
 
         expectThrows(TaskCancelledException.class, future);
 
+        logger.info("--> waiting for all ongoing tasks to complete within a reasonable time");
+        safeGet(clusterAdmin().prepareListTasks().setActions(TEST_TASK_ACTION.name() + "*").setWaitForCompletion(true).execute());
+
         logger.info("--> checking that test tasks are not running");
         assertEquals(0, clusterAdmin().prepareListTasks().setActions(TEST_TASK_ACTION.name() + "*").get().getTasks().size());
     }