Add **kwargs to all unit test model creation helper functions.
Review Request #2273 — Created Oct. 8, 2021 and updated
Our own `TestCase` class provides a number of helper functions for easily creating models. Some of these took a `**kwargs` argument, which was passed in to the model's constructor, allowing arbitrary fields to be provided, but not all. This change adds support for `**kwargs` in all the helpers that were missing it. Testing Done: All unit tests pass. Reviewed at https://reviews.reviewboard.org/r/11716/