Two instances of the test suite cannot be run concurrently.

[Raphaël Badin]

Django generates a fixed name (
https://docs.djangoproject.com/en/dev/topics/testing/#the-test-database) for the test database. It is generated by adding a prefix ('test_') to the NAME of the default database defined in the settings (DATABASES['default']['NAME']). This prevents many instances of the test suite from being run concurrently.

[Raphaël Badin (rvb)]

In fact since we create a local database for each instance, this should not be a problem to run multiple instances of the test suite provided they are run not from the same working tree.