gradereport_grader: Sorting the grader report table returns duplicate results when using pages.

Note: These testing instructions test for regression. The nature of the database sorting means that it is not reliably reproducible.
Note: This should be tested in all four DBs

1. Create a course.
2. Enrol at least 13 students
3. Create an assignment activity and name it as "assign1".
4. Set the Students per page to 3 at Administration > Grade administration > Setup > Preferences: Grader report.
5. Go to Grader report page at Administration > Grade administration > Grader Report.
6. Click the Turn editing on button.
7. Click the First name column sort to sort the table.
8. Enter 50 as the grade for the first 11 students. Leave the others as blank.
9. Make sure you are still on Page 1.
10. Check that you can sort by each of:
    1. firstname
    2. lastname
    3. email
    4. idnumber
    5. "assignment1" column
11. Click and sort using the "assignment1" (descending order).
12. On page 1, record the name of the students.
13. Click Page 2 and so on...
    1. The returned results should be unique and should not contain duplicates (no repeating of returned results in the pages).