lava: Apply LAVA 2021.05 workaround to aum tests as well

Followup on commit 4e9a8b1d to apply the fix on the snippet rebooting devices during the Apertis Update Manager tests, which are currently failing since they can't detect the prompt after the kernel has booted.

Test job: https://lava.collabora.co.uk/scheduler/job/4324414