[refactor/handler-test] Separate mock handler interfaces from the original file
[refactor/handler-test] Separate mock handler interfaces from the original file

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