test: wait a bit before stopping/killing service

test: wait a bit before stopping/killing service

Otherwise, when stopping the service, the last command may not be
started yet, and the service manager may not send SIGTERM signal to the
last command, but send SIGKILL on timeout.

===
May 21 08:23:24 test19-exit-cgroup.sh[437]: + disown
May 21 08:23:24 test19-exit-cgroup.sh[438]: + sleep infinity
May 21 08:23:24 test19-exit-cgroup.sh[437]: + systemd-notify --ready
May 21 08:23:24 test19-exit-cgroup.sh[437]: + sleep infinity
May 21 08:23:24 test19-exit-cgroup.sh[441]: + systemctl stop one
May 21 08:23:24 test19-exit-cgroup.sh[443]: + sleep infinity
(snip)
May 21 08:23:24 systemd[1]: one.service: Changed running -> stop-sigterm
May 21 08:23:24 systemd[1]: Stopping one.service - /tmp/test19-exit-cgroup.sh "systemctl stop one"...
May 21 08:23:24 systemd[1]: Received SIGCHLD from PID 441 (systemctl).
May 21 08:23:24 systemd[1]: Child 437 (bash) died (code=killed, status=15/TERM)
May 21 08:23:24 systemd[1]: one.service: Child 437 belongs to one.service.
May 21 08:23:24 systemd[1]: one.service: Main process exited, code=killed, status=15/TERM (success)
May 21 08:23:24 systemd[1]: Child 439 (bash) died (code=killed, status=15/TERM)
May 21 08:23:24 systemd[1]: one.service: Child 439 belongs to one.service.
May 21 08:23:24 systemd[1]: Child 441 (systemctl) died (code=killed, status=15/TERM)
May 21 08:23:24 systemd[1]: one.service: Child 441 belongs to one.service.
May 21 08:23:24 systemd[1]: Child 442 (bash) died (code=killed, status=15/TERM)
May 21 08:23:24 systemd[1]: one.service: Child 442 belongs to one.service.
(snip)
May 21 08:24:54 systemd[1]: one.service: State 'stop-sigterm' timed out. Killing.
May 21 08:24:54 systemd[1]: one.service: Killing process 443 (sleep) with signal SIGKILL.
May 21 08:24:54 systemd[1]: one.service: Changed stop-sigterm -> stop-sigkill
May 21 08:24:54 systemd[1]: Received SIGCHLD from PID 443 (sleep).
May 21 08:24:54 systemd[1]: Child 443 (sleep) died (code=killed, status=9/KILL)
May 21 08:24:54 systemd[1]: one.service: Child 443 belongs to one.service.
May 21 08:24:54 systemd[1]: one.service: Control group is empty.
May 21 08:24:54 systemd[1]: one.service: Failed with result 'timeout'.
May 21 08:24:54 systemd[1]: one.service: Service restart not allowed.
May 21 08:24:54 systemd[1]: one.service: Changed stop-sigkill -> failed
May 21 08:24:54 systemd[1]: one.service: Job 738 one.service/stop finished, result=done
May 21 08:24:54 systemd[1]: Stopped one.service - /tmp/test19-exit-cgroup.sh "systemctl stop one".
May 21 08:24:54 systemd[1]: one.service: Unit entered failed state.
May 21 08:24:54 systemd[1]: one.service: Releasing resources...
===

Fixes #32947.