test: add shutdown test

Wraps nspawn to be able to use pexpect. The test logs in on the console
and runs screen. In one screen window it types in shutdown commands and
checks whether a wall message was sent to the other.

test/units/testsuite-69.service

@@ -0,0 +1,7 @@
+# SPDX-License-Identifier: LGPL-2.1-or-later
+[Unit]
+Description=TEST-69-SHUTDOWN
+
+[Service]
+Type=oneshot
+ExecStart=/usr/lib/systemd/tests/testdata/units/%N.sh