test: make the systemd-run calls synchronous

Otherwise we might be checking results of such calls before they even
finish, causing nasty races like:

```
[   15.656530] testsuite-43.sh[303]: + su testuser -s /bin/sh -c 'XDG_RUNTIME_DIR=/run/user/$UID exec "$@"' -- sh systemd-run --user --unit=test-unprotected-home -P touch /home/testuser/works.txt
...
[   15.757744] testsuite-43.sh[324]: Running as unit: test-unprotected-home.service
[   15.775611] systemd[296]: Started /usr/bin/touch /home/testuser/works.txt.
[   15.783597] testsuite-43.sh[303]: + test -e /home/testuser/works.txt
[   15.787542] systemd[296]: test-unprotected-home.service: Succeeded.
...
[   15.787684] systemd[1]: Received SIGCHLD from PID 303 (bash).
[   15.787790] systemd[1]: Child 303 (bash) died (code=exited, status=1/FAILURE)
[   15.787881] systemd[1]: testsuite-43.service: Child 303 belongs to testsuite-43.service.
[   15.788040] systemd[1]: testsuite-43.service: Main process exited, code=exited, status=1/FAILURE
[   15.788224] systemd[1]: testsuite-43.service: Failed with result 'exit-code'.
[   15.788333] systemd[1]: testsuite-43.service: Service will not restart (restart setting)
[   15.788421] systemd[1]: testsuite-43.service: Changed start -> failed
[   15.788790] systemd[1]: testsuite-43.service: Job 160 testsuite-43.service/start finished, result=failed
[   15.788995] systemd[1]: Failed to start testsuite-43.service.
```

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