Enable Block Transformer Registry in tests

lms/djangoapps/course_api/blocks/tests/test_api.py

lms/djangoapps/course_api/blocks/tests/test_forms.py

lms/djangoapps/course_api/blocks/tests/test_serializers.py

lms/djangoapps/course_api/blocks/tests/test_views.py

lms/djangoapps/courseware/tests/test_discussion_module.py

lms/envs/test.py

.../core/djangoapps/content/block_structure/tests/helpers.py

.../djangoapps/content/block_structure/tests/test_signals.py