[test.broker] Fix small issue with error duplication
[test.broker] Fix small issue with error duplication

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