Unit Test Integration
By default, South's syncdb command will also apply migrations if it's run in non-interactive mode, which includes when you're running tests - it will run every migration every time you run your tests.
If you want the test runner to use syncdb instead of migrate - for example, if your migrations are taking way too long to apply - simply set SOUTH_TESTS_MIGRATE = False in settings.py.
South's own unit tests
South has its own set of unit tests; these will also be run when you run ./manage.py test. They do some fiddling with Django internals to set up a proper test environment; it's non-destructive, but if it's fouling up your own tests please submit a ticket about it.
You can also set SKIP_SOUTH_TESTS=True in settings.py to stop South's tests running, should they be causing issues.