Several recent failed runs show that the test is still racy in two ways:
1) Sometimes it takes a while before the PID file is created, leading
to:
```
[ 10.950540] testsuite-47.sh[308]: ++ cat /leakedtestpid
[ 10.959712] testsuite-47.sh[308]: cat: /leakedtestpid: No such file or directory
[ 10.959824] testsuite-47.sh[298]: + leaked_pid=
```
2) Again, sometimes we check the leaked PID before the unit is actually
stopped, leading to a false negative:
```
[ 18.099599] testsuite-47.sh[346]: ++ cat /leakedtestpid
[ 18.116462] testsuite-47.sh[333]: + leaked_pid=342
[ 18.117101] testsuite-47.sh[333]: + systemctl stop testsuite-47-repro
...
[ 20.033907] testsuite-47.sh[333]: + ps -p 342
[ 20.080050] testsuite-47.sh[351]: PID TTY TIME CMD
[ 20.080050] testsuite-47.sh[351]: 342 ? 00:00:00 sleep
[ 20.082040] testsuite-47.sh[333]: + exit 42
```
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
Frantisek Sumsal