tests: regenerate x509 test certificates The old x509 test certificates were using cryptographic settings that are ancient by today's standards, namely 512 bit RSA keys. To put things in perspective, browsers have been dropping support for 1024 bit RSA keys. I think it is important that tests match the realities of the times. And 2048 bit RSA keys with SHA-2 hashing are what the world is moving to. This patch replaces all the x509 certificates with new versions using modern best practices. In addition, the docs for generating the keys have been updated, as the existing docs left out a few steps, namely how to generate certs that were not active yet or expired.