code: def test_stoppable_thread(self):
      stop_event = threading.Event()
      thread = StoppableThread(stop_event=stop_event)
      self.assertFalse(thread.is_stopped())
      thread.stop()
      self.assertTrue(stop_event.is_set())
      self.assertTrue(thread.is_stopped())
  
imports: import threading
  import unittest
  from pfrl.utils import StoppableThread
label_desc: Use image node_id 1 for calling the TestStoppableThread obj's underlying member method code with example usage: obj.test_stoppable_thread() without return types