The integration-test-setup calls require StateDirectory= but some
tests override the test unit used which then won't have StateDirectory=
so let's move StateDirectory= into the dropin as well to avoid this
issue.
If we're not running interactively, there's no point in the features
from integration-test-setup.sh which are intended for interactive
development and debugging so lets skip adding it in that case.
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.
