Create tests for atom feeds

[Elrond]

We need some tests for our atom feeds.

How deeply the tests go, is left to the one implementing the tests.
But IMHO each tested feed should at least have one entry.

[Elrond]

Hi Tsyesika, as you work on pump and activity streams, could you at least comment on this?

[Jessica Tallon]

In the near future the content posted to Mediagoblin won't all be public, in fact by default it won't be public. This will require changes to the atom feeds we have now. I have spoken to Elrond, as people might still use and love the old style atom feeds they won't be removed in favour of the new pump style Activity Streams 1.0 feeds however they will need modifying to only show content listed as public.

The tests will probably just add some media which is targeted at "public" and then the feed checked to see if they're available, once the permission model is implemented there probably should also be a test to verify non-public media should not be added to the atom feed.

[Ben Sturmfels]

I added some basic tests for feeds while re-implementing on top of the feedgenerator library (previous feed classes were dropped from recent Werkzeug).