code: def check_leak(self, request, client):
      assert fiber.active_children() == []
      pre_workers = client.containers.list()
      yield
      post_workers = client.containers.list()
      assert pre_workers == post_workers
  
imports: import fiber
  import multiprocessing
  import time
  import pytest
  import docker
  import fiber.config
  from fiber.docker_backend import Backend
  from fiber.util import find_ip_by_net_interface
  import psutil
  import ipaddress
  import socket
label_desc: Use image node_id 1 for calling the TestDockerBackend obj's underlying member method code with example usage: obj.check_leak(request, client) without return types