Django's list of applied migrations persist between individual unit

tests, which causes state leakage issues, breaking some tests that are

sensitive to the migration history.
This change introduces a new MigrationsTestsMixin class, which handles

removing the list of applied test migrations before running a test. It

makes use of a new unrecord_applied_migrations, which can unrecord

specific migration names or all for an app.
To improve performance, both record_applied_migrations and

unrecord_applied_migrations now operate on the MigrationRecorder's

queryset, rather than using the utility functions, so that we can

manipulate more than one entry in one operation instead of several.

Unit tests pass on all supported versions of Django.