tests: Enable RSA SHA1 certs for testing against older OpenSSH
The OpenSSH 7.4 or 7.6 in Ubuntu and CentOS 7 does not support SHA2
RSA certificates and libssh automatically falls back to SHA1, which
is not allowed by default.
Signed-off-by: Jakub Jelen <firstname.lastname@example.org>
Reviewed-by: Anderson Toshiyuki Sasaki <email@example.com>