tests: Add test case for T191
Add a test case to verify that the server returns the correct signature
when it negotiated an RSA signature algorithm for the host key different
from the one it prefers (e.g. when the client prefers ssh-rsa over
rsa-sha2-256 and rsa-sha2-512).
Signed-off-by: Anderson Toshiyuki Sasaki <firstname.lastname@example.org>
Reviewed-by: Andreas Schneider <email@example.com>