test: add known broken test for good In-Reply-To / bad References

The current scheme of choosing the replyto (i.e. the default parent
for threading purposes) does not work well for mailers that put
the oldest Reference last.

test/T510-thread-replies.sh

@@ -209,4 +209,18 @@ End of search results.
 EOF
 test_expect_equal_file EXPECTED OUTPUT
 
+test_begin_subtest "trusting reply-to (tree view)"
+test_subtest_known_broken
+test_emacs '(notmuch-tree "id:B00-root@example.org")
+	    (notmuch-test-wait)
+	    (test-output)
+	    (delete-other-windows)'
+cat <<EOF > EXPECTED
+  2016-06-17  Alice                 ┬►root message                                        (inbox unread)
+  2016-06-18  Alice                 ╰┬►child message                                      (inbox unread)
+  2016-06-18  Alice                  ╰─►grand-child message                               (inbox unread)
+End of search results.
+EOF
+test_expect_equal_file EXPECTED OUTPUT
+
 test_done

test/corpora/threading/parent-priority/cur/child

@@ -0,0 +1,11 @@
+From: Alice <alice@example.org>
+To: Daniel <daniel@example.org>
+Subject: child message
+Message-ID: <B01-child@example.org>
+In-Reply-To: <B00-root@example.org>
+References:  <B00--root@example.org>
+Date: Fri, 17 Jun 2016 22:14:41 -0400
+
+This is a normal-ish reply, and has both a references header and an
+in-reply-to header.
+

test/corpora/threading/parent-priority/cur/grand-child

@@ -0,0 +1,10 @@
+From: Alice <alice@example.org>
+To: Daniel <daniel@example.org>
+Subject: grand-child message
+Message-ID: <B01-grand-child@example.org>
+In-Reply-To: <B01-child@example.org>
+References:  <B01-child@example.org> <B00-root@example.org>
+Date: Fri, 17 Jun 2016 22:24:41 -0400
+
+This has the references headers in the wrong order, with oldest first.
+Debbugs does this.

test/corpora/threading/parent-priority/cur/root

@@ -0,0 +1,7 @@
+From: Alice <alice@example.org>
+To: Daniel <daniel@example.org>
+Subject: root message
+Message-ID: <B00-root@example.org>
+Date: Thu, 16 Jun 2016 22:14:41 -0400
+
+This message has no reply-to