Expose a more generic tests/ volume instead of screenshots/

Follow up on discussion https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/20085#note_85165572

In this case I think that making the ~/tmp a placeholder for more QA artifacts and exposing this as artifacts/ with artifacts/screenshots and artifacts/logs makes sense for me 👍

Or instead of artifacts/ we can use tests/ or just qa/.

cc @grzesiek