kselftests for memory.oom.group

[jgkamat]

Here is an amended patch which fixes those issues.

Please let me know if you see anything else that could be improved.

Changes since the last patchset:

- Use if statement to check for empty string in cgroup_util.c
- Cleanup and return properly when failing to kill process
  in test_memcg_oom_group_score_events

Thanks,
-Jay