tests: Add regression test for mirroring with project.refs
Description
This MR fulfils the task described in #667 (closed)
Unfortunately, when reverting 68ef69e4 (source.py: Fix re-instantiation), the tests don't fail.
Changes proposed in this merge request:
- Add a regression test that fetches a source with known ref under {inline, project.refs} {no mirror, mirror, mirror with wrong alias}
This merge request is related to issue: #667 (closed) It does not, in its current form, close the issue, as it does not actually catch the regression it was written for.