rose suite-hook --shutdown: run with --kill --now

Now run `cylc shutdown` with `--kill --now`. This stops the suite
immediately. Otherwise, it may still be waiting for defunct jobs that
are in the submitted or running status.

Modify the `rose-suite-hook/00-shutdown.t` test to have a 2nd long running task to see if the hook triggered by the failure of the 1st task would cause the suite to shut down promptly or not.

Also some general tidy up.