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have been revoked.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>

test/T350-crypto.sh

@@ -453,6 +453,7 @@ y
     | gpg --no-tty --quiet --import
 output=$(notmuch show --format=json --verify subject:"test signed message 001" \
     | notmuch_json_show_sanitize \
+    | sed -e 's/"key-\(revoked\|missing\)"/"key-revoked"/g' \
     | sed -e 's|"created": [1234567890]*|"created": 946728000|')
 expected='[[[{"id": "XXXXX",
  "match": true,