test: Add a way to quickly iterate on an integration test

test: Add a way to quickly iterate on an integration test

Rebuilding the integration test every time is very slow. Let's
introduce a way to iterate on an integration test without rebuilding
the image every time. By making a btrfs snapshot before we run the
integration test, we can then systemctl soft-reboot after running
the test to restore the rootfs to a pristine state before running
the test again.

As /run/nextroot will get nuked on reboot or soft-reboot, we introduce
a tmpfiles snippet to make sure it is recreated every (soft-)reboot
and adapt the existing tests to deal with this new symlink.