Hi,
Sorry, but I know very little if anything at all about Behat, to be truthful, however I found this which may help you?
https://docs.moodle.org/dev/Running_acceptance_test
If nothing else you may find that some of the people who contributed to that document may have a better understanding of the problems you have encountered.
If no-one answers your questions here, it might be better to ask in the General Forum and at the same time add a link to this Forum post. That way you might get more people reading it and offering you advice!
Hope you found this helpful?
Cheers
Mary