selftest/cgroup: Make test_cpuset_prs.sh deal with pre-isolated CPUs

Since isolated CPUs can be reserved at boot time via the "isolcpus"
boot command line option, these pre-isolated CPUs may interfere with
testing done by test_cpuset_prs.sh.

With the previous commit that incorporates those boot time isolated CPUs
into "cpuset.cpus.isolated", we can check for those before testing is
started to make sure that there will be no interference.  Otherwise,
this test will be skipped if incorrect test failure can happen.

As "cpuset.cpus.isolated" is now available in a non cgroup_debug kernel,
we don't need to check for its existence anymore.

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