Tests are supposed to be non-interactive, and ssh-keygen is asking for
a passphrase when creating a key.  Let's set an empty passphrase to
avoid the prompt.

Signed-off-by: Cleber Rosa <crosa@redhat.com>
Reviewed-by: Beraldo Leal <bleal@redhat.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20210203172357.1422425-6-crosa@redhat.com>
[PMD: Reword description per Alex Bennée comment]
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>