Make split modulestore the default store in django_utils.py

Description

/edx-platform/common/lib/xmodule/xmodule/modulestore/tests/django_utils.py contains modulestore configuration used by a lot of our unit tests. In that configuration, the "old mongo" modulestore is listed before the split modulestore, so "old mongo" will be the default modulestore.

We should switch that ordering, and possibly create a configuration that maintains old mongo as the default for some number of tests.

When doing this work, one interesting test is lms/djangoapps/courseware/management/commands/tests/test_dump_course.py, which is currently running the same test in 3 different configurations-
1) the default as defined by our env files (as of is the split modulestore)
2) mixed with all 3 stores (includes XML), but old mongo being the default
3) mixed without XML, but old mongo being the default

Steps to Reproduce

None

Current Behavior

None

Expected Behavior

None

Reason for Variance

None

Release Notes

None

User Impact Summary

None

Assignee

Unassigned

Reporter

ChristinaR

Labels

Reach

None

Impact

None

Platform Area

None

Customer

None

Partner Manager

None

URL

None

Contributor Name

None

Groups with Read-Only Access

None

Actual Points

None

Category of Work

None

Platform Map Area (Levels 1 & 2)

None

Platform Map Area (Levels 3 & 4)

None

Priority

Unset
Configure