Specify DEFAULT_AUTO_FIELD in tests settings
Specify DEFAULT_AUTO_FIELD in tests settings

Avoids Django 3.2 warning:

```
schedule.Rule: (models.W042) Auto-created primary key used when not defining a primary key type, by default 'django.db.models.AutoField'.
        HINT: Configure the DEFAULT_AUTO_FIELD setting or the ScheduleConfig.default_auto_field attribute to point to a subclass of AutoField, e.g. 'django.db.m
odels.BigAutoField'.
```

llazzaro/django-scheduler,llazzaro/django-scheduler,llazzaro/django-scheduler