Reddit object creation in the test suite is messy

[gsidebo]

We seem to have a few different implementations for creating fake reddit objects in our tests. Each one is suited to a limited set of use cases, and none of them are very friendly in terms of configuration/customization. Some examples:

1. Dummy SimpleNamespace object with properties that can't be specifically set: https://github.com/mitodl/open-discussions/blob/master/fixtures/reddit.py#L115
2. Simple class that takes kwargs and sets properties: https://github.com/mitodl/open-discussions/blob/master/channels/factories.py#L78 (this seems to tack on some extra properties that don't exist on reddit objects)
3. Setting specific reddit properties on a Mock object: https://github.com/mitodl/open-discussions/blob/master/channels/api_test.py#L624

We should make some honest factory classes for these objects. The LOC impact from these different implementations is pretty severe, and the test suite is much harder to understand because of it.