Quiz reports should use enroled users code, not get_users_by_capability

Create a course, make visible and add 3 groups. Create at least 4 students (s1 to s4). Enrol these students on the course. Put the students into two groups (leave one group empty). Create quiz1, with visible groups. Add an Essay question (Write something here...) - for manual marking Add a True/False question (Choose one...) - for auto marking Login as s1, start quiz1, answer questions correctly. Login as s2, answer one question correctly. Login as s3, start quiz but do not answer any questions. Login as s4, do not start quiz. Login as admin. Check all pages of the quiz reports (Administration > Quiz administration > Results) and every option with each page. Be sure to test features on the Quiz Grades and Responses reports, such as selecting attempts and deleting them, or re-grading attempts. The above will prove that nothing is broken. Now to prove that this change actually works it is necessary to start with a course created by admin/tool/generator/maketestcourse.php with 50,000 enrolled users (look at the code to work out the setting required), and then do all the steps above on that course. Without these code changes many of the quiz reports do not work.