Replace 'multi_db = True' with 'databases = {alias for alias in connections}'

common/lib/xmodule/xmodule/modulestore/tests/django_utils.py

lms/djangoapps/ccx/tests/test_field_override_performance.py

lms/djangoapps/courseware/tests/test_model_data.py

lms/djangoapps/courseware/tests/test_submitting_problems.py

lms/djangoapps/courseware/tests/test_user_state_client.py

lms/djangoapps/coursewarehistoryextended/tests.py

...x/core/djangoapps/content_libraries/tests/test_runtime.py