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)

Inizia leggendo il modulo del repository e il relativo esempio di repository, inclusa la classe SimpleRepository, quindi esamina la struttura dei test esistente. Aggiungi la copertura per l’aggiornamento di un client, il download di un file e l’aggiornamento dei metadati del repository dalla versione N alla versione N+M, verificando che i metadati aggiornati siano corretti.