human
Here a sensible value to return is given the following:
Complete the below
def connected_server_and_client(case, server_factory, client_factory):

    ''''Create a server and client connected to that server.
    :param case: The test case that will handle cleanup.
    :param IProtocolFactory server_factory: The factory for the server protocol.
    :param IProtocolFactory client_factory: The factory for the client protocol.
    :return: A deferred that fires when the client has connected.
    .. todo:
    Figure out what a sensible value to return is. The existing caller doesn\'t
    use the return value.'
    '''
assistant
#Here a sensible value to return is.
    def connect_client(listening_port):
        return TCP4ClientEndpoint(reactor, '127.0.0.1', listening_port.getHost().port).connect(client_factory)
    return listen(case, TCP4ServerEndpoint(reactor, 0), server_factory).addCallback(connect_client)