test: switch default test store to the split store

It's long past time that the default test modulestore was Split,
instead of Old Mongo. This commit switches the default store and
fixes some tests that now fail:
- Tests that didn't expect MFE to be enabled (because we don't
  enable MFE for Old Mongo) - opt out of MFE for those
- Tests that hardcoded old key string formats
- Lots of other random little differences

In many places, I didn't spend much time trying to figure out how to
properly fix the test, and instead just set the modulestore to Old
Mongo.

For those tests that I didn't spend time investigating, I've set
the modulestore to TEST_DATA_MONGO_AMNESTY_MODULESTORE - search for
that string to find further work.

openedx/features/enterprise_support/tests/test_utils.py

@@ -20,6 +20,7 @@ from opaque_keys.edx.keys import CourseKey, UsageKey
 
 from common.djangoapps.student.models import CourseEnrollment
 from common.djangoapps.student.tests.factories import UserFactory
+from lms.djangoapps.course_home_api.toggles import COURSE_HOME_USE_LEGACY_FRONTEND
 from openedx.core.djangolib.testing.utils import skip_unless_lms
 from openedx.features.enterprise_support.tests import FEATURES_WITH_ENTERPRISE_ENABLED
 from openedx.features.enterprise_support.tests.factories import (
@@ -638,6 +639,7 @@ class TestCourseAccessed(SharedModuleStoreTestCase, CompletionWaffleTestMixin):
             }
         )
 
+    @override_waffle_flag(COURSE_HOME_USE_LEGACY_FRONTEND, active=True)
     def test_course_accessed_for_visit_course_home(self):
         """
         Test that a visit to course home does not fall under course access