Add plugin and test to reproduce failing tests since c9fe9af

With all the change and revert mess of the HOOK_BASEFS_UP tests started
failing due to usr.so and sys.so failed to set up their conditions.  It
then became clear that boomisc.so cleaned up /run ... with the revert
we got service_register() to be called before global_rlimit[] was setup
causing services registered by plugins, before conf_init(), to crash.

This plugin and test is here to ensure we catch this problem in case of
any future refactor.

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

test/testserv.sh

@@ -0,0 +1,15 @@
+#!/bin/sh
+# Verify service readiness notification
+
+set -eu
+#set -x
+TEST_DIR=$(dirname "$0")
+#DEBUG=true
+
+# shellcheck source=/dev/null
+. "$TEST_DIR/tenv/lib.sh"
+
+sleep 1
+retry 'assert_norestart serv' 1 1
+
+return 0