test-lfs: add tests to force server error path coverage The tests are somewhat fragile in that the extension that forces the errors is counting how many times some of the functions are being called, so it depends heavily on the content of the repo. Maybe we can do something clever like load an extension on the client, and have it send over instructions in the HTTP header how to fail. (I'm trying to avoid killing and restarting the server, because Windows seems to have issues with doing that a lot.) But I'd rather fix issues than polish tests before the freeze.