selftest/cgroup: Clean up and restructure test_cpuset_prs.sh

jira NONE_AUTOMATION
Rebuild_History Non-Buildable kernel-5.14.0-570.17.1.el9_6
commit-author Waiman Long <[email protected]>
commit b2b2b4d

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 <[email protected]>
	Signed-off-by: Tejun Heo <[email protected]>
(cherry picked from commit b2b2b4d)
	Signed-off-by: Jonathan Maple <[email protected]>

Author: PlaidCat