Will/peer rendering test cleanup

[TIM-424](https://edx-wiki.atlassian.net/browse/TIM-424)

Remove unnecessary submission_uuid in self-assessment tests.
Add test cases for all peer assessment rendering paths.

I didn't change the peer rendering code itself because we're likely going to need to change it again when [TIM-254](https://edx-wiki.atlassian.net/browse/TIM-254) and [TIM-253](https://edx-wiki.atlassian.net/browse/TIM-253) land.  This should give us a better safety net when we do that.

@ormsbee or @stephensanchez