* Add basic test coverage for /auth API
* Check /auth API is called from an add-on
Currently the /auth API is only available for add-ons. Return 403
for calls not originating from an add-on.
* Handle bad json in auth API
Use the API specific JSON load helper which raises an APIError. This
causes the API to return a 400 error instead of a 500 error when the
JSON is invalid.
* Avoid redefining name 'mock_check_login'
* Update tests/api/test_auth.py
