test: verify sysupdate invokes the notification callout directory

test: verify sysupdate invokes the notification callout directory

Extend TEST-72-SYSUPDATE with a check that, after a successful update,
systemd-sysupdate connects to every socket linked into
/run/systemd/sysupdate/notify/ and invokes
io.systemd.SysUpdate.Notify.OnCompletedUpdate(). A tiny recorder socket is
hooked into that directory; it captures the request and replies with success.
We assert the recorded call carries the expected method, version and resource
list, and that a subsequent no-op update emits no notification.