expand code in test/cert_utils.go to support generating certificates for testing cosign to TSA server (over mTLS)

[dmitris]

Description

This is a follow-up on #3052 and specifically #3052 (comment). We need to add code used to generate testdata certificates in the cosign repo. I would suggest to expand the library package test/cert_utils.go to have functions to generate certificates, and then add a new directory with a package main program as a "driver" to use those functions to actually to generate the certificates (as well as the private key etc.) Other ways of doing the same are possible - for example, write a script that will generate everything using the appropriate openssl commands, etc.