tests: Add S/MIME messages to protected-headers corpus

tests: Add S/MIME messages to protected-headers corpus

These sample messages are taken directly from the Protected Headers
draft:

https://www.ietf.org/id/draft-autocrypt-lamps-protected-headers-02.html

Note that this commit doesn't strictly pass the common git pre-commit
hook due to introducing some trailing whitespace.  That's just the
nature of the corpus, though.  We should have that trailing
whitespace, so I've made this commit with --no-verify.

Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>