xpi: update short rsa key cert test
xpi: update short rsa key cert test

new cert generated with:

openssl req -x509 -key test_short_rsa.key -keyform PEM -out cert.pem -days 36500 -addext keyUsage=keyCertSign -addext extendedKeyUsage=codeSigning

mozilla-services/autograph,mozilla-services/autograph,mozilla-services/autograph