/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