Add "upstream" autopkgtest

This runs the test/TEST* upstream integration tests in QEMU and nspawn.

We need to modify test-functions a bit: We don't want to run this against a
built source tree (which we don't even have for a distro test run), but against
the installed packages.

Skip TEST-06-SELINUX for now as this does not work in Ubuntu (even after
install policycoreutils): The test expects a /usr/libexec/selinux/hll/pp file
which does not exist.