Fix message_tester.py
Fix message_tester.py

Make skill optional and disable expected_dialog if missing.

Dark5ide/mycroft-core,linuxipho/mycroft-core,Dark5ide/mycroft-core,linuxipho/mycroft-core,MycroftAI/mycroft-core,forslund/mycroft-core,forslund/mycroft-core,MycroftAI/mycroft-core