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.
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