selftest/cgroup: Clean up and restructure test_cpuset_prs.sh

Cleaning up the test_cpuset_prs.sh script and restructure some of the
functions so that a new test matrix with a different cgroup directory
structure can be added in the next patch.

Signed-off-by: Waiman Long <longman@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>