code: def test_twenty_worker(self):
      self.add_master()
      cluster_monitor = ClusterMonitor("localhost:{}".format(self.port))
  
      for _ in range(20):
          self.add_worker(n_cpu=1)
  
      check_flag = False
      for _ in range(10):
          if 20 == len(cluster_monitor.data["workers"]):
              check_flag = True
              break
          time.sleep(10)
      self.assertTrue(check_flag)
  
      self.remove_ten_workers()
  
      check_flag = False
      for _ in range(10):
          if 10 == len(cluster_monitor.data["workers"]):
              check_flag = True
              break
          time.sleep(10)
      self.assertTrue(check_flag)
  
      self.remove_all_workers()
      # check if the number of workers drops to 0
      check_flag = False
      for _ in range(10):
          if 0 == len(cluster_monitor.data["workers"]):
              check_flag = True
              break
          time.sleep(10)
      self.assertTrue(check_flag)
  
imports: import unittest
  import parl
  from parl.remote.monitor import ClusterMonitor
  import time
  import threading
  from parl.remote import exceptions
  from parl.utils.test_utils import XparlTestCase
  import os
  import signal
label_desc: Use image node_id 2 for calling the TestClusterMonitor obj's underlying member method code with example usage: obj.test_twenty_worker() without return types