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](https://bugzilla.redhat.com/show_bug.cgi?id=1102732). The test
  passes, which indicates that the bug has been fixed.
* `api/v2/roles` suffers from [bugzilla bug 1118015](https://bugzilla.redhat.com/show_bug.cgi?id=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?)
* Creating tests for this URL has revealed another error, which is documented in
  [bugzilla bug 1129785](https://bugzilla.redhat.com/show_bug.cgi?id=1129785). Also, this PR contains tests targeting that specific
  bug.
`make test-docstrings` doesn't show any errors. What's up with that?