message: update test_participant_heartbeat to use message_loop
diff: @@ -20,7 +20,13 @@ from xain.grpc.coordinator import (
  Participants,
  monitor_heartbeats,
  )
  -from xain.grpc.participant import end_training, heartbeat, rendezvous, start_training
  +from xain.grpc.participant import (
  + StateRecord,
  + end_training,
  + message_loop,
  + rendezvous,
  + start_training,
  +)
  # Some grpc tests fail on macos.
  # `pytestmark` when defined on a module will mark all tests in that module.
  @@ -132,8 +138,9 @@ def test_monitor_heartbeats_remove_participant(_mock_sleep, _mock_event):
  def test_participant_heartbeat(mock_heartbeat_request, _mock_sleep, _mock_event):
  channel = mock.MagicMock()
  terminate_event = threading.Event()
  + st = StateRecord()
  - heartbeat(channel, terminate_event)
  + message_loop(channel, st, terminate_event)
  # check that the heartbeat is sent exactly twice
  mock_heartbeat_request.assert_has_calls([mock.call(), mock.call()])
  