workhorse: Move test repo to a directory not gitignored

What does this MR do and why?

Before this change, the test repo symlink is under a directory that's in workhorse/.gitignore. As a result, when gitlab-foss pulls changes from gitlab project, it doesn't include the symlink. This causes workhorse tests in gitlab-foss project to fail.

To fix it, the test repo is moved to a directory that is not in .gitignore.

This also fixes the issue when make clean is called and it deletes the symlink.

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.

Related to #372390 (closed)