selftest/cgroup: Clean up and restructure test_cpuset_prs.sh

JIRA: https://issues.redhat.com/browse/RHEL-83455

commit b2b2b4d
Author: Waiman Long <longman@redhat.com>
Date:   Sun, 30 Mar 2025 17:52:47 -0400

    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>

Signed-off-by: Waiman Long <longman@redhat.com>

Author: Waiman-Long