The base TestCase used for our unit tests offer a number of useful

functions for creating instances of the models we need. This expands

that support for many of them and fleshes out documentation for the ones

gaining new parameters.
The comment creation methods now allow for custom issue statuses and

arbitrary model attributes to be set.
create_review() no longer takes a username argument (which was

unused in the function). The only caller of this has been updated to use

user instead. This also allow for arbitrary model attributes.
create_status_update() now accepts a review argument.

All unit tests pass.