Re-applied bugfix #24214, but behind a waffle flag. The change gates timed exams for audit learners. We would like to limit access to timed exams to verified learners. However, some of the instructors would like to make their content free for all users, including graded content/timed exams (hence the Feature-Based Enrollment "FBE" exemption). As a result, we will be rolling the gated timed exams functionality out behind a waffle flag now, add the FBE exemption when we figure out how, and gradually turn the waffle flag on for everyone.