message: tfdbg: de-flake and re-enable a test in source_remote_test
  For some reason, the debug grpc server startup is sometimes
  delayed in Python3. Adding polling of test server to fix that.
diff: @@ -48,7 +48,8 @@ class SendTracebacksTest(test_util.TensorFlowTestCase):
  test_util.TensorFlowTestCase.setUpClass()
  (cls._server_port, cls._debug_server_url, cls._server_dump_dir,
  cls._server_thread,
  - cls._server) = grpc_debug_test_server.start_server_on_separate_thread()
  + cls._server) = grpc_debug_test_server.start_server_on_separate_thread(
  + poll_server=True)
  cls._server_address = "localhost:%d" % cls._server_port
  (cls._server_port_2, cls._debug_server_url_2, cls._server_dump_dir_2,
  cls._server_thread_2,
  @@ -201,8 +202,7 @@ class SendTracebacksTest(test_util.TensorFlowTestCase):
  with self.assertRaises(ValueError):
  self._server.query_source_file_line(tf_trace_file_path, 0)
  - # TODO(b/124381153): Re-enable this test when not flaky.
  - def DISABLED_testSendEagerTracebacksToSingleDebugServer(self):
  + def testSendEagerTracebacksToSingleDebugServer(self):
  this_func_name = "testSendEagerTracebacksToSingleDebugServer"
  send_traceback = traceback.extract_stack()
  send_lineno = line_number_above()
  