Add tests for memory.oom.group

Add tests for memory.oom.group for the following cases:
- Killing all processes in a leaf cgroup, but leaving the
  parent untouched
- Killing all processes in a parent and leaf cgroup
- Keeping processes marked by OOM_SCORE_ADJ_MIN alive when considered
  for being killed by the group oom killer.

Signed-off-by: Jay Kamat <jgkamat@fb.com>
Acked-by: Roman Gushchin <guro@fb.com>
Signed-off-by: Shuah Khan (Samsung OSG) <shuah@kernel.org>