selftests: cgroup: Simplify task self migration

Simplify task migration by being oblivious about its PID during
migration. This allows to easily migrate individual threads as well.
This change brings no functional change and prepares grounds for thread
granularity migrating tests.

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