repository module: Add tests

[jku]

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)

[jku]

This should include the uploader as well. The basic end-to-end test for all examples would be:

1. run repository/repo in one process
2. wait until http server is up
3. run uploader/uploader (in another process) to initialize with trust-on-first-use, create a delegation and to add a target
4. wait until upload has finished
5. run client/client (in 3rd process or reusing the uploader process) to initialize with trust-on-first-use and download target

This would be useful even without any other checks (like checking which metadata versions are used): if the client can download the file and the contents are correct, that's a really good indication the examples are running pretty well.