Remove test dependency on django guardian

[jmsmkn]

Description

An ObjectPermissionBackend is added which is similar to the one
in guardian.backends.ObjectPermissionBackend for test support.
The differences are that:

• Permissions can only be assigned to groups
• The object permissions are stored in a defaultdict
• Permissions can only be assigned using app_label.codename

This allows DRF to be released without waiting for django-guardian
to update, refs #3427, refs #7927, refs django-guardian/django-guardian#747.

[carltongibson]

I suspect this is probably a good idea. We check that we didn't break anything from the contract point of view. If anything actually were to break with Guardian, I imagine we'd hear about it.

[jmsmkn]

The list view object permissions tests were moved to https://github.com/rpkilby/django-rest-framework-guardian/blob/master/tests/test_permissions.py when the DjangoObjectPermissionsFilter was extracted from DRF. That might be the logical place to add the other object permissions tests to check on integration with Guardian itself.

[johnthagen]

This may no longer be necessary as django-guardian just published a release: #7927 (comment)

[tomchristie]

I don't think we need this anymore, but thanks for your time on it all the same.