grade-school: Catch false positives due to test case being sorted already (#467)

From Python 3.6 on, dictionaries retain the order in which items are
added. If the test case puts the items into the tested class in sorted
order, even a naive implementation (one that does no sorting) can pass
by giving back all items in the original order.

exercises/grade-school/grade_school_test.py

@@ -45,8 +45,8 @@ class SchoolTest(unittest.TestCase):
         students = [(3, ("Kyle", )), (4, ("Christopher", "Jennifer", )),
                     (6, ("Kareem", ))]
 
-        for grade, students_in_grade in students:
-            for student in students_in_grade:
+        for grade, students_in_grade in students[::-1]:
+            for student in students_in_grade[::-1]:
                 self.school.add(student, grade)
 
         result = self.school.sort()