Support for contextual tuples in model tests

[mbarralo]

Currently the contextual tuples don't seem to be supported in the model tests, although it is possible using API requests with curl, Postman, etc.

Example:

  - name: User can see account from specific branch
    check:
    - user: user:anne
      object: account:1
      contextual_tuples:
        - user: user:anne
          relation: assignee
          object: branch:lisbon
      assertions:
        can_view: true