refactor: migrate ENABLE_COURSE_FILENAME_CCX_SUFFIX off FEATURES-as-dict

`common/djangoapps/util/file.py` read the flag via
`settings.FEATURES.get('ENABLE_COURSE_FILENAME_CCX_SUFFIX', False)`. The
setting is defined only in lms/envs/common.py, but this reader lives in
common/ and is reachable from CMS, so use
`getattr(settings, 'ENABLE_COURSE_FILENAME_CCX_SUFFIX', False)` to avoid an
AttributeError under Studio settings. Convert the test's whole-dict
`@override_settings(FEATURES={...})` to
`@override_settings(ENABLE_COURSE_FILENAME_CCX_SUFFIX=True)`. Verified the
CCX-suffix tests pass under both lms.envs.test and cms.envs.test.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>