[Draft] Setup behat testing

Created by: mbovel

This is a work-in-progress, not ready yet to review or merge.

The goal of this PR is to setup behat tests and the necessary config to run them both locally and on the CI.

It's mainly cherry-picking what we need from https://github.com/moodlehq/moodle-docker. See also https://moodledev.io/general/development/tools/behat.