Fix the setup in tests/Sanity/testsuite
    
    "koji download-build" is a wrong tool to download artifacts being
    tested. They are freshly scratch-built and hosted in a local repo
    called "test-artifacts" unknown to koji. Using "dnf download"
    directly and limited to that specific repo is a correct way to
    reach them.
    
    I'm adding a fallback using the repo's local files just in case
    "dnf" suffers some temporary hiccup.