test: signature verification during decryption (session keys)

test: signature verification during decryption (session keys)

When the user knows the signer's key, we want "notmuch show" to be
able to verify the signature of an encrypted and signed message
regardless of whether we are using a stashed session key or not.

I wrote this test because I was surprised to see signature
verification failing when viewing some encrypted messages after
upgrading to GPGME 1.13.0-1 in debian experimental.

The added tests here all pass with GPGME 1.12.0, but the final test
fails with 1.13.0, due to some buggy updates to GPGME upstream: see
https://dev.gnupg.org/T3464 for more details.

While the bug needs to be fixed in GPGME, notmuch's test suite needs
to make sure that GMime is doing what we expect it to do; i was a bit
surprised that it hadn't caught the problem, hence this patch.

I've fixed this bug in debian experimental with gpgme 1.13.0-2, so the
tests should pass on any debian system.  I've also fixed it in the
gpgme packages (1.13.0-2~ppa1) in the ubuntu xenial PPA
(ppa:notmuch/notmuch) that notmuch uses for Travis CI.

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