Remove gitlab.com clone dependency from Workhorse tests

Fixes #337179 (closed)

The Workhorse test suite fetches data from a real git repository in some of its tests. In CI this means we need to clone gitlab-org/gitlab-test.git. This MR changes those tests to use the embedded copy of gitlab-org/gitlab-git-test instead. That way, we cannot have CI jobs fail because of network issues.