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/courseware/tests/test_submitting_problems.py

@@ -7,10 +7,12 @@ Integration tests for submitting problem responses and getting grades.
 
 import json
 import os
+from datetime import datetime
 from textwrap import dedent
-
 from unittest.mock import patch
+
 import ddt
+import pytz
 from django.conf import settings
 from django.contrib.auth.models import User  # lint-amnesty, pylint: disable=imported-auth-user
 from django.db import connections
@@ -208,7 +210,11 @@ class TestSubmittingProblems(ModuleStoreTestCase, LoginEnrollmentTestCase, Probl
                 parent_location=self.chapter.location,
                 display_name=name,
                 category='sequential',
-                metadata={'graded': True, 'format': section_format, 'due': '2013-05-20T23:30'}
+                metadata={
+                    'graded': True,
+                    'format': section_format,
+                    'due': datetime(2013, 5, 20, 23, 30, tzinfo=pytz.utc),
+                },
             )
         elif reset:
             section = ItemFactory.create(