cgroup/cpuset: Add test_cpuset_v1_hp.sh

Add a simple test to verify that an empty v1 cpuset will force its tasks
to be moved to an ancestor node. It is based on the test case documented
in commit 76bb5ab8 ("cpuset: break kernfs active protection in
cpuset_write_resmask()").

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