class_docstr: Terminates once it receives the first ping() call.
code: def ping(self):
      self._has_ping.set()
      return "pong"
  
imports: import datetime
  import sys
  import threading
  import time
  from unittest import mock
  from absl.testing import absltest
  import courier
  from launchpad.launch.test_multi_threading import address_builder
  from launchpad.nodes.courier import node
label_desc: Use image node_id 2 for calling the Server obj's underlying member method code with example usage: obj.ping() and returns: str