Accept "key-missing" from a signature from a revoked key

Accept "key-missing" from a signature from a revoked key

We have traditionally expected a signature to show up as "revoked"
when the signing key is revoked.  However, GnuPG's recent fix to avoid
a denial of service against legitimate signatures appears to have
changed the status of signature verification from keys which happen to
have been revoked.

See https://bugs.debian.org/1098995 and https://dev.gnupg.org/T7547

This change makes the test suite a little bit less brittle while we
look for a resolution from upstream.  It should probably also be
backported to debian unstable unless a notmuch release to unstable is
imminent.

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