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.

lms/djangoapps/course_api/blocks/tests/pacts/views.py

@@ -7,7 +7,7 @@ from django.http import JsonResponse
 from django.views.decorators.csrf import csrf_exempt
 from django.views.decorators.http import require_POST
 from opaque_keys.edx.keys import CourseKey
-from xmodule.modulestore.tests.django_utils import TEST_DATA_SPLIT_MODULESTORE, ModuleStoreIsolationMixin
+from xmodule.modulestore.tests.django_utils import ModuleStoreIsolationMixin
 from xmodule.modulestore.tests.factories import CourseFactory
 
 from common.djangoapps.student.models import CourseEnrollment
@@ -16,9 +16,6 @@ from common.djangoapps.student.tests.factories import CourseEnrollmentFactory
 
 class ProviderState(ModuleStoreIsolationMixin):
     """ Provider State Setup """
-
-    MODULESTORE = TEST_DATA_SPLIT_MODULESTORE
-
     def clean_db(self, user, course_key):
         """ Delete objects from SQL DB and clean mongodb instance """