Parallel test mode not working on MacOS with recent versions of Python 3

[Brandon Navra]

Python 3.8 on MacOS has changed the default start method for the multiprocessing module from fork to spawn: ​https://docs.python.org/3/library/multiprocessing.html#contexts-and-start-methods.

When running tests with the --parallel flag, this causes the worker processes to fail with django.core.exceptions.AppRegistryNotReady: Apps aren't loaded yet. as they no longer have a copy of the parent memory state. It can also cause the workers to fail to find the cloned dbs ( {{django.db.utils.OperationalError: FATAL: database "xxx_1" does not exist}} ) as the db test prefix is missing.

I have attached a patch which changes django.test.runner._init_worker (the worker initialiser for ParallelTestSuite) to run django.setup() and set the db name to one with the test_ prefix.