refactor: migrate EMBARGO test overrides off FEATURES-as-dict

The EMBARGO production readers (lms/urls.py, openedx/core/djangoapps/embargo/api.py)
already read flat settings.EMBARGO; two verify_student view tests still set the flag via
@patch.dict(settings.FEATURES, {'EMBARGO': True}). Convert them to
@override_settings(EMBARGO=True) (matching the already-migrated decorator elsewhere in
the file) so no settings.FEATURES usage remains here.

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