selftests: cgroup: Minor code reorganizations

No functional change intended, these small changes are merged into one
commit and they serve as a preparation for an upcoming new testcase.

Signed-off-by: Michal Koutný <mkoutny@suse.com>
Reviewed-by: Waiman Long <longman@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>