Add API tests for api/v2/roles

* Add a full suite of tests targeting the "Role" entity.
* `EntityTestCase.test_get_status_code` targets bugzilla bug 1102732. The test
  passes, which indicates that the bug has been fixed.
* `api/v2/roles` suffers from bugzilla bug 1118015, and this is addressed in the
  updated test suite.
* The existing entity had an incorrect API path, and this has been fixed.
  (perhaps it has changed?)