tests/nested/core20/kernel-failover: add test for failed refresh of uc20 kernel
Merge pull request #9208 from anonymouse64/feature/uc2-kernel-failed-refresh-rolls-back
This exercises both the snapd logic for falling back properly and cleaning up
after a failed refresh, as well as pc-gadget grub.conf logic that rollbacks are
handled properly.
Eventually with resealing there should be additional things we check for here,
but for now it's sufficient to show that the kernel got rolled back successfully
and that we don't leave any intermediate state behind about the attempted
refresh.
boot: make MockUC20Device use a model and MockDevice more realistic
We will need to add Model to Device for UC20 cases, prepare for that
by forcing MockUC20Device to use one and produce output consistent
with it.
Make also the behavior of MockDevice a bit more consistent/realistic
though it has limited use for UC20, it allowed for mode != "run"
with HasModeenv false for example.