The certificate for /etc/shibboleth/sso-test.pem can be found at https://sso-test.rwth-aachen.de/metadata/sso.pem. You can also retrieve this via wget and place it directly at the destination:
wget https://sso-test.rwth-aachen.de/metadata/sso.pem -O /etc/shibboleth/sso-test.pem |
chown shibd:shibd /etc/shibboleth/sso-test.pem
chmod 664 /etc/shibboleth/sso-test.pem