Convert test-loop.sh into minitest

In order to create test files/folders, this could help:

FileUtils.touch('zdt-0.tar.gz', mtime: Time.now - 3600 * 24 * 30)
=> ["zdt-0.tar.gz"]
File.mtime(Dir.glob(zd_dl_glob)[0])
=> 2021-09-13 12:33:56.081038 +0200

Not sure, if a different "namespace" via a different zd_dl_glob pattern would be useful there. Maybe better use a different --downloads-dir/zd_dl_dir.

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