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/program_enrollments/api/tests/test_linking.py

@@ -10,6 +10,7 @@ from django.test import TestCase
 from edx_django_utils.cache import RequestCache
 from opaque_keys.edx.keys import CourseKey
 from testfixtures import LogCapture
+from xmodule.modulestore.tests.django_utils import ModuleStoreTestCase
 
 from common.djangoapps.student.api import get_course_access_role
 from common.djangoapps.student.roles import CourseStaffRole
@@ -363,7 +364,7 @@ class TestLinkProgramEnrollments(TestLinkProgramEnrollmentsMixin, TestCase):
         self._assert_user_enrolled_in_program_courses(self.user_2, self.program, self.fruit_course, self.animal_course)
 
 
-class TestLinkProgramEnrollmentsErrors(TestLinkProgramEnrollmentsMixin, TestCase):
+class TestLinkProgramEnrollmentsErrors(TestLinkProgramEnrollmentsMixin, ModuleStoreTestCase):
     """ Tests for linking error behavior """
 
     def test_program_enrollment_not_found__nonexistant(self):