Fix flaky CSV test

The test compares the raw CSV string directly, but the actual order of
the objects returned from the database is unspecified and sometimes gives
a test failure:

MarksGradersControllerTest#test_: An authenticated admin should download
a csv on GET :
download_grader_students_mapping.
[/home/su/code/Markus/test/functional/marks_graders_
controller_test.rb:114]:
<"machinist_student1,machinist_ta1,machinist_ta2\nmachinist_student2\nmachinist_student
3\nmachinist_student4\nmachinist_student5\n"> expected but was
<"machinist_student3\nmachinist_student1,machinist_ta1,machinist_ta2\nmachinist_student
2\nmachinist_student4\nmachinist_student5\n">.

This can be fixed by comparing the set of rows.

Tested:
- rake test:units
- rake test:functionals