Add missing test

+# This file is part of GNU Mailutils. -*- Autotest -*-
+# Copyright (C) 2016 Free Software Foundation, Inc.
+#
+# GNU Mailutils is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License as
+# published by the Free Software Foundation; either version 3, or (at
+# your option) any later version.
+#
+# GNU Mailutils is distributed in the hope that it will be useful, but
+# WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+# General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with GNU Mailutils.  If not, see <http://www.gnu.org/licenses/>.
+
+AT_SETUP([MU_PARSEOPT_EXIT_ERROR])
+AT_KEYWORDS([parseopt MU_PARSEOPT_EXIT_ERROR parseopt24])
+AT_CHECK([
+PARSEOPT_DEFAULT
+MU_PARSEOPT_EXIT_ERROR=64 parseopt -X
+],
+[64],
+[],
+[parseopt: unrecognized option '-X'
+])
+AT_CLEANUP