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>

tools/testing/selftests/cgroup/cgroup_util.h

@@ -40,3 +40,4 @@ extern int get_temp_fd(void);
 extern int alloc_pagecache(int fd, size_t size);
 extern int alloc_anon(const char *cgroup, void *arg);
 extern int is_swap_enabled(void);
+extern int set_oom_adj_score(int pid, int score);