Give systemd a chance to process the stop event before checking if the
PID has indeed leaked. This should fix the intermittent test fails in CI
even with a fixed systemd version, like this one:
```
Apr 08 10:22:09 testsuite-47.sh[345]: ++ cat /leakedtestpid
Apr 08 10:22:09 testsuite-47.sh[334]: + leaked_pid=342
Apr 08 10:22:09 testsuite-47.sh[334]: + systemctl stop testsuite-47-repro
Apr 08 10:22:10 testsuite-47.sh[334]: + ps -p 342
Apr 08 10:22:10 testsuite-47.sh[348]: PID TTY TIME CMD
Apr 08 10:22:10 testsuite-47.sh[348]: 342 ? 00:00:00 sleep
Apr 08 10:22:10 testsuite-47.sh[334]: + exit 42
```
Followup to 197298ff9f
