selftests: cgroup: Add cpuset migrations testcase

Add a separate testfile to verify treating permissions when tasks are
migrated on cgroup v2 hierarchy between cpuset cgroups.

In accordance with v2 design, migration should be allowed based on
delegation boundaries (i.e. cgroup.procs permissions) and does not
depend on the migrated object (i.e. unprivileged process can migrate
another process (even privileged) as long as it remains in the original
dedicated scope).

Signed-off-by: Michal Koutný <mkoutny@suse.com>
Signed-off-by: Tejun Heo <tj@kernel.org>