Rename system_test to get_test_system

common/lib/xmodule/xmodule/tests/__init__.py

common/lib/xmodule/xmodule/tests/test_annotatable_module.py

common/lib/xmodule/xmodule/tests/test_capa_module.py

common/lib/xmodule/xmodule/tests/test_combined_open_ended.py

common/lib/xmodule/xmodule/tests/test_conditional.py

common/lib/xmodule/xmodule/tests/test_error_module.py

common/lib/xmodule/xmodule/tests/test_html_module.py

common/lib/xmodule/xmodule/tests/test_peer_grading.py

common/lib/xmodule/xmodule/tests/test_progress.py

common/lib/xmodule/xmodule/tests/test_self_assessment.py

common/lib/xmodule/xmodule/tests/test_util_open_ended.py

common/lib/xmodule/xmodule/tests/test_xml_module.py

lms/djangoapps/courseware/tests/__init__.py

lms/djangoapps/courseware/tests/test_video_xml.py