test: use systemd inside client test rootfs if available

test: use systemd inside client test rootfs if available

Some tests (for example test 10 on ubuntu:devel) show this warning:

```
System has not been booted with systemd as init system (PID 1). Can't operate.
Failed to connect to system scope bus via local transport: Host is down
```

This is caused by calling `systemctl --failed` in the client test rootfs
where systemd is not available/used.

Use systemd inside the client test rootfs if available to allow querying
the failed services from the initrd.