Further improvements to Quiz behat steps

MDL-63185 (part of MDL-62610) made some awesome improvements to the generator and Behat 'Given' steps for mod_quiz and questions.

However, it somewhat fudged the issue of question behaviours. (Behaviours are plugins, but the current implementation just hard-codes it to work with the common behaviours.) That made sense at the time, but at some point we should improve it.