Add tests for DjangoObjectType only/exclude_fields

[samdroid-apps]

I was planning to write a new feature to rename fields on a DjangoObjectType, because I wanted my foreign key relationships to have nice reverse names. So I started working on that by writing these tests.

However, I eventually opened the "tests/models.py" field and saw how the "related_name" can be used to rename the relationship. So I didn't need to write the feature any more, but tests are still cool :)

[coveralls]

Coverage remained the same at 93.75% when pulling 46048cd on LearntEmail:rename_fields into 1139507 on graphql-python:master.

[syrusakbary]

Thanks for the contribution!