Tails

My understanding is that the tests you wrote about deleting old SquashFS diffs currently mostly replicate the tests we already have in iuk.git, but they don't exercise the part that I'm actually concerned about, i.e. the behaviour that can only be exercised via full-blown integration testing: deleting old SquashFS diffs that are in use by the running system. I'm assuming that when we do "Tails is fooled to think a 2.1~test squashfs delta is installed", no such SquashFS delta is really union-mounted.

And when you'll implement that, as I had requested in 2763439ddf07cf0fb4eec091a40135e7dda1a67f on the (now gone) feature/11131-endless-upgrade branch, I'd like us to test that any old SquashFS diff is gone after shutting down the system: while we're using them we can merely unlink them but we cannot be sure they have really been deleted. Depending on fine details about our shutdown procedure (and in particular the crazy things we do for the memory erasure feature), such deletion might be effectively applied to the filesystem before it's unmounted cleanly, or things can go wrong, I dunno and I really would like to know and have a safeguard in case we ever break it (e.g. when migrating to overlayfs, which will force me to rewrite great parts of our shutdown procedure).

Minor nitpicking: please s/squashfs/SquashFS/ in various places :)

The test suite needs to be updated wrt. #15282: FindFailed: TailsUpgraderOkButton.png when the upgrader tells the IUK download is complete.