This removes the use of the "random" qtype for random questions from the
quiz module. Instead, the quiz structure now idenfies slots containing
random questions using a `random` boolean field on the slot.
This includes some amendments to statistics classes in the core_question
namespace, as they ultimately get their question data from
mod/quiz/report/reportlib.php
It also introduces a proper (if limited) way to add random questions in
behat tests.
Mark Johnson
fe29d48b87