drop certtool from testing scripts

[The-Mule]

Currently, we are using certtool in testing scripts (setup.sh) to generate certificates. We can replace this by openssl tools and drop dependency on gnutls-utils (RHEL / Fedora).

Please notice that there is no specific problem with certtool or gnutls-utils. It is just not necessary to use it when openssl is available anyway and having less dependencies feels always better.

[simo5]

I would love to reduce our dependencies and that should allow us to handle the CKK_EC_EDWARDS case with 3.1 behavior which gnutls certtool can't currently handle.