SSHUpload{Pack,Archive}: fix timeout tests

Fixes #2222 (closed)

We incorrectly assumed that in these timeout scenarios, the server would remain quiet until the client sent anything on the stdin stream. Due to timing variations in go test -race, it became apparent that the server speaks first: our assumption was wrong.

This change updates the tests to look for a failure of the server side command, and ignore server output. That way it does not matter what the server says.