test: wait a bit after stopping the test service

test: wait a bit after stopping the test service

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 197298ff9fc930de450330095cc5b67d165d0801