$args =
{
 message_type => MQTT_SUBSCRIBE,
 message_id => 1,
 topics => [ [ 'testing' => MQTT_QOS_AT_MOST_ONCE ] ],
}

82 0c 00 01  00 07 74 65    73 74 69 6e  67 00

Subscribe/at-least-once 1 testing/at-most-once