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/core/djangoapps/models/tests/test_course_details.py

@@ -10,12 +10,12 @@ from pytz import UTC
 from unittest.mock import patch  # lint-amnesty, pylint: disable=wrong-import-order
 
 from django.conf import settings
+from xmodule.modulestore import ModuleStoreEnum
+from xmodule.data import CertificatesDisplayBehaviors
+from xmodule.modulestore.tests.django_utils import TEST_DATA_MONGO_AMNESTY_MODULESTORE, ModuleStoreTestCase
+from xmodule.modulestore.tests.factories import CourseFactory
 
 from openedx.core.djangoapps.models.course_details import ABOUT_ATTRIBUTES, CourseDetails
-from xmodule.modulestore import ModuleStoreEnum  # lint-amnesty, pylint: disable=wrong-import-order
-from xmodule.data import CertificatesDisplayBehaviors  # lint-amnesty, pylint: disable=wrong-import-order
-from xmodule.modulestore.tests.django_utils import ModuleStoreTestCase  # lint-amnesty, pylint: disable=wrong-import-order
-from xmodule.modulestore.tests.factories import CourseFactory  # lint-amnesty, pylint: disable=wrong-import-order
 
 EXAMPLE_CERTIFICATE_AVAILABLE_DATE = datetime.date(2020, 1, 1)
 
@@ -25,6 +25,7 @@ class CourseDetailsTestCase(ModuleStoreTestCase):
     """
     Tests the first course settings page (course dates, overview, etc.).
     """
+    MODULESTORE = TEST_DATA_MONGO_AMNESTY_MODULESTORE
 
     def setUp(self):
         super().setUp()