test/crypto: test message with rfc822 attachment.

This is intended to help track down a display problem in the emacs
front end

test/T357-index-decryption.sh

@@ -226,6 +226,7 @@ output=$(notmuch dump | LC_ALL=C sort)
 expected='#= simple-encrypted@crypto.notmuchmail.org index.decryption=failure
 #notmuch-dump batch-tag:3 config,properties,tags
 +encrypted +inbox +unread -- id:basic-encrypted@crypto.notmuchmail.org
++encrypted +inbox +unread -- id:encrypted-rfc822-attachment@crypto.notmuchmail.org
 +encrypted +inbox +unread -- id:encrypted-signed@crypto.notmuchmail.org
 +encrypted +inbox +unread -- id:simple-encrypted@crypto.notmuchmail.org'
 test_expect_equal \

test/corpora/crypto/encrypted-rfc822-attachment

@@ -0,0 +1,52 @@
+Content-Type: multipart/encrypted; protocol="application/pgp-encrypted"; boundary="===============9060418334135509864=="
+MIME-Version: 1.0
+From: Notmuch test suite <test_suite@notmuchmail.org>
+To: test_suite@notmuchmail.org
+Subject: testing encrypted rfc822 attachments
+Date: Sat, 03 Jul 2021 16:00:02 -0300
+Message-ID: <encrypted-rfc822-attachment@crypto.notmuchmail.org>
+User-Agent: alot/0.9.1
+
+--===============9060418334135509864==
+MIME-Version: 1.0
+Content-Transfer-Encoding: 7bit
+Content-Type: application/pgp-encrypted; charset="us-ascii"
+
+Version: 1
+--===============9060418334135509864==
+MIME-Version: 1.0
+Content-Transfer-Encoding: 7bit
+Content-Type: application/octet-stream; charset="us-ascii"
+
+-----BEGIN PGP MESSAGE-----
+
+hIwDxE023q1UqxYBBACGKSDv5/rcwScSf9n33cZZPPxltQgxkDaClMGY2DARgebE
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+n+wqLQE7kllgI32E4U9B
+=YlXg
+-----END PGP MESSAGE-----
+
+--===============9060418334135509864==--