test: wait for underlying .device unit being active before invoking systemd-mount

test: wait for underlying .device unit being active before invoking systemd-mount

Fixes following failure:
===
May 17 04:12:04 TEST-74-AUX-UTILS.sh[2684]: + systemd-mount --owner=testuser /dev/loop0 /tmp/tmp.DVQdo2ou53/mnt
(snip)
May 17 04:15:04 systemd[1]: dev-loop0.device: Job dev-loop0.device/start timed out.
May 17 04:15:04 systemd[1]: dev-loop0.device: Job 5812 dev-loop0.device/start finished, result=timeout
May 17 04:15:04 systemd[1]: Timed out waiting for device dev-loop0.device - /dev/loop0.
May 17 04:15:04 systemd[1]: tmp-tmp.DVQdo2ou53-mnt.mount: Job 5804 tmp-tmp.DVQdo2ou53-mnt.mount/start finished, result=dependency
May 17 04:15:04 systemd[1]: Dependency failed for tmp-tmp.DVQdo2ou53-mnt.mount - /tmp/tmp.DVQdo2ou53/mnt.
May 17 04:15:04 systemd[1]: tmp-tmp.DVQdo2ou53-mnt.mount: Job tmp-tmp.DVQdo2ou53-mnt.mount/start failed with result 'dependency'.
May 17 04:15:04 systemd[1]: systemd-fsck@dev-loop0.service: Job 5805 systemd-fsck@dev-loop0.service/start finished, result=dependency
May 17 04:15:04 systemd[1]: Dependency failed for systemd-fsck@dev-loop0.service - File System Check on /dev/loop0.
May 17 04:15:04 systemd[1]: systemd-fsck@dev-loop0.service: Job systemd-fsck@dev-loop0.service/start failed with result 'dependency'.
May 17 04:15:04 systemd[1]: dev-loop0.device: Job dev-loop0.device/start failed with result 'timeout'.
(snip)
May 17 04:15:04 systemd-mount[2856]: A dependency job for tmp-tmp.DVQdo2ou53-mnt.mount failed. See 'journalctl -xe' for details.