code: def testSingleClusterResolver(self):
      base_cluster_spec = server_lib.ClusterSpec(
          {
              "ps": ["ps0:2222", "ps1:2222"],
              "worker": [
                  "worker0:2222",
                  "worker1:2222",
                  "worker2:2222",
              ],
          }
      )
      simple_resolver = cluster_resolver.SimpleClusterResolver(
          base_cluster_spec
      )
      union_resolver = cluster_resolver.UnionClusterResolver(
          simple_resolver
      )
  
      expected_proto = """
      job { name: 'ps' tasks { key: 0 value: 'ps0:2222' }
                       tasks { key: 1 value: 'ps1:2222' } }
      job { name: 'worker' tasks { key: 0 value: 'worker0:2222' }
                           tasks { key: 1 value: 'worker1:2222' }
                           tasks { key: 2 value: 'worker2:2222' } }
      """
      actual_cluster_spec = union_resolver.cluster_spec()
      self._verifyClusterSpecEquality(
          actual_cluster_spec, expected_proto
      )
  
imports: from tensorflow.python.client import session
  from tensorflow.python.distribute.cluster_resolver import cluster_resolver
  from tensorflow.python.eager import context
  from tensorflow.python.framework import config
  from tensorflow.python.framework import test_util
  from tensorflow.python.platform import test
  from tensorflow.python.training import server_lib
label_desc: Use image node_id 2 for calling the UnionClusterResolverTest obj's underlying member method code with example usage: obj.testSingleClusterResolver() without return types