selftest: the systemd.sh block fails when the selftest runs as root #4

Run as root (in a container, say), the block "== systemd.sh: installing, removing and checking units" fails on every check. The scratch machine is then owned by root, and systemd.sh rightly refuses to install units for a machine owned by root (SERVICE_USER would be root). CI runs as an ordinary user, so it stays green, but anyone running the suite as root sees 14 false failures.

What to build: under root, give the scratch machine an ordinary owner (for instance chown -R nobody on $SM, if that user exists), or skip the block with a line that says why — a skip must say so, never pass silently.

Where to look: platform/bin/selftest.sh, the SM="$WORK/units-machine" block.

Done when: docker run --rm -v "$PWD":/src ubuntu:24.04 … running the selftest as root shows no failures in that block.