[test-router] Write tests for HandleDown method
[test-router] Write tests for HandleDown method

mgranberry/ttn,LoRaWanSoFa/ttn,jvanmalder/ttn,LoRaWanSoFa/ttn,mgranberry/ttn,TheThingsNetwork/ttn,TheThingsNetwork/ttn,mgranberry/ttn,TheThingsNetwork/ttn,jvanmalder/ttn,jvanmalder/ttn