test: add testcase that verifies we can safely delete subcgroups owned by other users...

test: add testcase that verifies we can safely delete subcgroups owned by other users if we own the parent

This is a test for the previous commits: we create an unpriv, delegated cgroup in
--user mode, then create a subcgroup that is owned by some other user
(to mimic the case where an unpriv user got a userns with delegated UIDs
assigned), and then try to stop the unit. traditionally this would fail,
because our unpriv systemd --user instance can't remove the subcrroup
owned by someone else. With the earlier patches this is addressed.