snap: use refresh hook to self-test on upgrade

[Nish Aravamudan]

There are now refresh hooks in snaps, and since we have a well-defined self-test (git-ubuntu.self-test), we can ensure that no user is upgraded to a broken snap by leveraging it in a refresh hook.

[Kyle Fazzari (kyrofa)]

Specifically, what you probably want is the `post-refresh` hook (as of 2.29). Basic informational docs are here:

    https://forum.snapcraft.io/t/supported-snap-hooks/3795

A few tutorials for how to use hooks with Snapcraft can be found here (it links to a second one on the `configure` hook): https://kyrofa.com/posts/snap-install-time-setup-the-install-hook .