Now that Resources don't share attributes, Fork doesn't have Project
attributes. That caused the merge fork test to fail because it no
longer had access to repository_http_location via Fork. Rather than
give Fork Project attributes directly, we add a project attribute to
Fork and populate it via the API at the end of the fabricate! method
after the fork has been created via the UI.

We also update the merge fork test to be slightly more concise and
consistent with newer tests by using perform with proc coercion
instead of act or perform with the full block syntax.