"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.
The test's Makefiles are not used in Fedora CI infrastructure. But are kept here for backward compatibility with traditional beakerlib test harness in RHEL.