Write the ssh key to disk rather than relying on /dev/stdin
    
    This is required to get the tests to pass in a container where /dev/stdin
    is not available inside the process running the tests.
    So rather than relying on /dev/stdin, we just write the key to disk in a
    temp folder that is cleaned up automatically once we're done checking if
    the ssh key is valid or not.
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>