Prevent creation of multiple test databases with test_* prefix (e.g. test_db, test_test_db, ...)

When there was exception during syncdb (for example import error in models.py file of your app), test database was created but syncdb threw exception, leaving connection.setting_dict with a new name of test database. Next test created new database with another test_ prefix, the whole process repeated itself. Now it only creates one database.