test: protected headers should work when both encrypted and signed.

test: protected headers should work when both encrypted and signed.

Up to this point, we've tested protected headers on messages that have
either been encrypted or signed, but not both.

This adds a couple tests of signed+encrypted messages, one where the
subject line is masked (outside subject line is "Subject Unavailable")
and another where it is not (outside Subject: matches inner Subject:)

See the discussion at
https://dkg.fifthhorseman.net/blog/e-mail-cryptography.html#protected-headers
for more details about the nuances between signed, stripped, and
stubbed headers.

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