Update test cert generation scripts to use SHA-256 by default

This cleans up the README file to clearly indicate which certificates are real world certificates, which are generated by hand / by other sources, and which are generated via script (and which script).

Additionally, several test certificates that were previously generated by hand and several test CRLSets that were hardcoded are now generated automatically by the scripts.

BUG=401365

Review URL: https://codereview.chromium.org/515583004

Cr-Commit-Position: refs/heads/master@{#297047}
9 files changed