tests: hup: rollback-altboot: replace while loop over SSH to speed up…

[rcooke-warwick]

… tests

The altboot test often hangs for a long time during the altboot test, after the DUT has booted into the old OS after the altboot failure - the check to see if the rollback systemd services have stopped hangs. This is an attempt to fix that, as I think there's some strange interaction going on with the while loop over ssh

Change-type: patch

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[klutchell]

I'm not convinced that this actually speeds up anything, since it needs to open a new SSH connection for each check of the service, but it is definitely cleaner code using existing functions!