I don't believe that the MX will use any specific VLAN for the test, it will just use the primary uplink for the test, in your case is WAN2. I am not sure what is the end goal here, but I just want to say that dashboard throughput is not really something that you can use as speed test data. If you want to test the speed, it is better to just use any device downstream of the MX (server or any computer), then you can use the server/computer IP to create your traffic shaping rule.