Added a test for #23617 -- UUID fields using uuid.uuid4 as the default

This new test checks that instances of fields.UUIDField, having the
default set to the uuid.uuid4 callable, will automatically generate a
valid UUID4 identifier, if one is not provided.

The test was created as an attempt to replicate [#23617](https://code.djangoproject.com/ticket/23617). It follows the related Django Developer Docs [example](https://docs.djangoproject.com/en/dev/ref/models/fields/#django.db.models.UUIDField).
Fixed in 7e06ec7