<p>To avoid a FileField with a custom storage using AWS S3 (django-storages) to try authenticating  when model is saved you can patch the save method of the instance of the storage. 
Eg.: 
<code>python
class MyTest(TestCase):
    @mock.patch('my_app.models.my_s3_boto_storage_instance.save')
    def test_create_my_model(self, save):
        pass  # test model creation</code></p>