Use a fake SSH client that runs commands locally to test remote path resolution, removing old backups, finding the last backup and the rsync arguments, plus the login paths with a fake paramiko client and real keys made by ssh-keygen.
Run real rsync into a temp dir to test hard links, keep, remove-before-backup, stray files in the backup dir and the old folder name format, and use a fake Popen to test how rsync return codes are handled.
Test how command line and config values are combined, the exit codes for an incomplete SSH setup and a bad config, and the user detection under sudo/doas (simulated by patching geteuid and the environment).
Configure pytest in pyproject.toml to import the repo copy, add a conftest that keeps tests out of the journal and desktop notifications, and test reading the config file and expanding paths.
Fuxino
