refactor: migrate MODE_CREATION_FOR_TESTING off FEATURES-as-dict

Setting is LMS-only (lms/envs/common.py) but the reader is in shared
common/djangoapps/course_modes/urls.py, so use getattr with the prior
False default. The test's setUp-level patch.dict becomes a class-level
@override_settings (trap: override_settings on setUp conflicts with
CacheIsolationMixin, and the class needs the flag set during URL reset).