test-cgroup-util: Check return values

test-cgroup-util: Check return values

The test is supposed to check a battery of cgroup helpers on each
process found but it doesn't literally check anything besides presence
of procfs. (One can visually check printed output only. Introduction in
aff38e74bd ("nspawn: suffix the nspawn cgroups with ".nspawn"").)

Make some assumptions about visible processes and turn the test into
testing that systemd helpers can deal with whatever process they find on
the SUT.

Reported by Coverity and
Fixes CID#1587767