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/grades/tests/base.py

@@ -4,14 +4,14 @@ Base file for Grades tests
 
 
 from crum import set_current_request
+from xmodule.modulestore.tests.django_utils import TEST_DATA_MONGO_AMNESTY_MODULESTORE, SharedModuleStoreTestCase
+from xmodule.modulestore.tests.factories import CourseFactory, ItemFactory
 
 from capa.tests.response_xml_factory import MultipleChoiceResponseXMLFactory
 from common.djangoapps.student.models import CourseEnrollment
 from common.djangoapps.student.tests.factories import UserFactory
 from lms.djangoapps.course_blocks.api import get_course_blocks
 from openedx.core.djangolib.testing.utils import get_mock_request
-from xmodule.modulestore.tests.django_utils import SharedModuleStoreTestCase  # lint-amnesty, pylint: disable=wrong-import-order
-from xmodule.modulestore.tests.factories import CourseFactory, ItemFactory  # lint-amnesty, pylint: disable=wrong-import-order
 
 from ..course_data import CourseData
 from ..subsection_grade_factory import SubsectionGradeFactory
@@ -21,6 +21,8 @@ class GradeTestBase(SharedModuleStoreTestCase):
     """
     Base class for some Grades tests.
     """
+    MODULESTORE = TEST_DATA_MONGO_AMNESTY_MODULESTORE
+
     @classmethod
     def setUpClass(cls):
         super().setUpClass()