Additional shard balancing changes for unit tests.

lms/djangoapps/bulk_email/tests/test_tasks.py

lms/djangoapps/ccx/tests/test_field_override_performance.py

lms/djangoapps/course_api/tests/test_serializers.py

lms/djangoapps/courseware/tests/test_entrance_exam.py

lms/djangoapps/courseware/tests/test_group_access.py

lms/djangoapps/courseware/tests/test_submitting_problems.py

lms/djangoapps/courseware/tests/test_views.py

lms/djangoapps/shoppingcart/tests/test_models.py

lms/djangoapps/shoppingcart/tests/test_views.py