Data generator enrol_user should let you specify role shortname

In writing unit tests you often want to enrol users to a course, and sometimes with a role. In that case the following pattern is almost universal:

$studentroleid = $DB->get_field('role', 'id', array('shortname' => 'student'), MUST_EXIST);

$generator->enrol_user($student1->id, $course->id, $studentroleid);

This is a waste of time/space - the enrol_user function should accept a role shortname directly (nobody cares if new unit tests do a few extra get_field calls as a result). The above code will then become the much nicer:

$generator->enrol_user($student1->id, $course->id, 'student');