repository module: Add tests

Once #2193 is merged, there is a repository module. The module is not really tested but should be.

It's entirely possible that the module can't really be effectively unit tested (as it's an ABC). But the repository example can be end-to-end tested and maybe even the SimpleRepository class within the example could be unit tested. I think testing the whole repository example is most critical.

I'm not sure what kind of things we should test but at least

• refreshing a client, downloading a file from the repository
• making sure the repository metadata actually changes, and that the updated metadata is correct (i.e. client that has metadata version N can successfully update to N+M)

Beginne damit, das Repository-Modul und sein Repository-Beispiel zu lesen, einschließlich der Klasse SimpleRepository, und untersuche anschließend die bestehende Teststruktur. Füge Testabdeckung für das Aktualisieren eines Clients, das Herunterladen einer Datei und das Aktualisieren von Repository-Metadaten von Version N auf N+M hinzu, wobei überprüft wird, dass die aktualisierten Metadaten korrekt sind.