tests: test refresh mixing essential and apps in hybrid systems

[miguelpires]

Check that on a hybrid system, refreshes of both essential snaps and apps and non-model bases don't make the apps wait for a reboot. In particular check that the refresh of the kernel, gadget and model base wait for a reboot. Check that apps that depend on the model base also wait for it to be refreshed and that apps that don't can completely refresh before the reboot.

Based on https://github.com/snapcore/snapd/pull/13925