treepkgdiff: Don't require a booted deployment

For the test suite, we're doing an upgrade offline of another OS root.
What we should maybe do is print the package diff from the merge
deployment, if not the booted one.

However, I think this is OK for now.
Import some basic testing infrastructure from OSTree

This is using the
https://wiki.gnome.org/Initiatives/GnomeGoals/InstalledTests model.

In order to run the tests, you will need to:
./configure --enable-installed-tests

Then:
gnome-desktop-testing-runner -p 0 rpm-ostree/
compose: Serialize treefile early

This way we don't emit a critical if we exit due to having already
cached content.