Add multi_3 test case for the DMS

[stae1102]

issue: #465

Test if the clients synchronize with each other by turning the server off and on again.

    let server_on_off_time = Duration::from_millis(1000);
    let server_on_off_repetition = 3;

    for _ in 0..server_on_off_repetition {
        let server = tokio::spawn(Dms::serve(
            Arc::clone(&server_dms),
            server_network_config.clone(),
        ));
        tokio::time::sleep(server_on_off_time).await;
        drop(server);
        tokio::time::sleep(server_on_off_time).await;
    }

In order to turn off and on the server, I just dropped server.