Make messaging system testable

[Christopher Allan Webber]

We should make it possible to test the messaging system.

If you look at mediagoblin/utils.py you can see what happens when
we \_activate\_testing() for some other things... ie, we insert
template context info like so:

::

        if TESTS_ENABLED:
            TEMPLATE_TEST_CONTEXT[template_path] = context

Maybe we should do something similar with messaging? Either we
should store in a dictionary like TEMPLATE\_TEST\_CONTEXT, maybe
TEST\_MESSAGES (which gets wiped by clear\_test\_buckets()) or we
should store in the session like request.session['test\_messages']
(should also get wiped by clear\_test\_buckets())

I'm assigning this to Caleb Davis who I think is a good fit for
it.