test: add known broken tests for recursive traversal of replies.

This reproduces the bug reported at [1]. The second test hints at the
solution, making reply return OwnedMessage objects.

[1]: id:87sfu6utxg.fsf@tethera.net

test/T392-python-cffi-notmuch.sh

@@ -0,0 +1,50 @@
+#!/usr/bin/env bash
+test_description="python bindings (notmuch test suite)"
+. $(dirname "$0")/test-lib.sh || exit 1
+
+if [ $NOTMUCH_HAVE_PYTHON3_CFFI -eq 0 -o $NOTMUCH_HAVE_PYTHON3_PYTEST -eq 0 ]; then
+    test_done
+fi
+
+add_email_corpus
+
+cat <<EOF > recurse.py
+from notmuch2 import Database
+def show_msgs(msgs, level):
+    print('{:s} {:s}'.format(' ' * level*4, type(msgs).__name__))
+    for msg in msgs:
+        print('{:s} {:s}'.format(' ' * (level*4+2), type(msg).__name__))
+        replies=msg.replies()
+        show_msgs(replies, level+1)
+db = Database(config=Database.CONFIG.SEARCH)
+msg=db.find("87ocn0qh6d.fsf@yoom.home.cworth.org")
+threads = db.threads(query="thread:"+msg.threadid)
+thread = next (threads)
+show_msgs(thread, 0)
+EOF
+
+test_begin_subtest "recursive traversal of replies (no crash)"
+test_subtest_known_broken
+test_python < recurse.py
+error=$?
+test_expect_equal "${error}" 0
+
+test_begin_subtest "recursive traversal of replies (output)"
+test_subtest_known_broken
+test_python < recurse.py
+tail -n 10 < OUTPUT > OUTPUT.sample
+cat <<EOF > EXPECTED
+   OwnedMessage
+     MessageIter
+   OwnedMessage
+     MessageIter
+       OwnedMessage
+         MessageIter
+   OwnedMessage
+     MessageIter
+   OwnedMessage
+     MessageIter
+EOF
+test_expect_equal_file EXPECTED OUTPUT.sample
+
+test_done