test-oomd-util: fix running in mkosi

When this test is run in mkosi, the previously tested cgroup that we write
xattrs into and the root cgroup are the same.

Since the root cgroup is a live cgroup anyways (vs. the test cgroups which are
remade each time) let's generate the expected preference values from reading
the xattrs instead of assuming it will be NONE.

(cherry picked from commit 080ca0d8)