[simulators.gateway] Extend forwarder tests
[simulators.gateway] Extend forwarder tests

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