Author: Marcus Boon
MDL-68388 core_grades: Use MUC for grade letters
In the get_grade_letters there is a static variable that is used
to cache grade letters, we should use MUC for this so that it is
reset properly between unit tests.
http://git.moodle.org/gw?p=moodle.git;a=commit;h=f1f5ba36ad7893e3fcc98a54804e1dbd22c3bea8
http://github.com/moodle/moodle/commit/f1f5ba36ad7893e3fcc98a54804e1dbd22c3bea8
+ 3.9 en [cachedef_grade_letters,core_cache]