v2020 ← v2022dev3 backport: lava: Apply LAVA 2021.05 workaround to aum tests as well

Backport tests/apertis-test-cases!310 (merged)

Followup on commit 593b3c13 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.