Prohibit non-admin reviewers from reviewing admin-flagged add-ons (bug 940756)

This looks weird, and not sure it's working the way it should. `assert` is a statement, not a function call... did you run this test?

Also, we're using the nose helpers for the moment, so that would be writting something like:
```python
ok_('public' in status.keys())
ok_('prelim' in status.keys())
```