d/t/test-common: Move inclusion of extensions when signing the certificate

When generating the certificate requests the req_extensions sections are relevant, when creating and signing the certificate with the CA using -x509 then x509_extensions is relevant. Add the passing of config to the signing step as we want the certificate to contain the basicConstraints with CA:false and extendedKeyUsage as well.

As per README.Debian the name(s) and purposes are ignore, but the intention was to include them as set in the role specific openssl.cnf configuration.