The spec has always been that add-ons with `PUBLIC` status could have a separate beta channel, to serve unreviewed updates of development versions. If we get a request for an update to a beta version, we serve updates to other beta versions, but only if the add-on has public status. That wasn't properly enforced in the previous code. It is now.

I'm not sure why that line is needed, though. The add-on's status is `UNREVIEWED` without it, but it should be `PUBLIC` according to the fixture.