Add test helpers for creating review request drafts and HTTP requests.
Review Request #1902 — Created April 9, 2020 and updated
Many of the unit tests in our codebase involve creating an `HttpRequest` and setting state on it (like users, a `LocalSite`, or a `ResolverMatch`), which is done through a `RequestFactory`. This requires a lot of annoying, repeated code. We also often need to create a `ReviewRequestDraft`, which is easier, but still requires more setup than is generally required for our other models. This change introduces new helpers for both. `create_http_request()` takes care of constructing the `HttpRequest` from the factory and setting various properties on it. Down the road, this could be updated to provide more capabilities like running middleware. `create_review_request_draft()` is a simple wrapper around `ReviewRequestDraft.create()`, which lets unit tests avoid the extra import. A few unit tests have been updated to use `create_http_request()`, to ensure that everything works as expected. Testing Done: Unit tests pass. Made use of these new helpers in upcoming unit tests. This is a backport of aeb5d6438ade5853e29da044a1dddcd0c4cb7df3 from release-4.0.x. Reviewed at https://reviews.reviewboard.org/r/10710/