Fix TasksIT#testTasksCancellation (elastic#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 elastic#109686

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

@@ -487,7 +487,6 @@ 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)
@@ -510,6 +509,9 @@ 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());
  }