[simulators.gateway] Write gateway.Stats() tests
[simulators.gateway] Write gateway.Stats() tests

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