[router] Add tests about gateway <-> router adapter listening goroutine
[router] Add tests about gateway <-> router adapter listening goroutine

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