Add and update some test keys and certificates

* Add trusted S/MIME test CA and replace self-signed S/MIME certificates
  with certificates certified by the test CA
* Add OpenPGP keys and S/MIME certificates with lower/same/higher validity
  than/as corresponding OpenPGP keys and S/MIME certificates
* Remove secret keys of S/MIME certificates of test recipients

GnuPG-bug-id: 5283