MAINTAINERS: add corresponding kselftests to cgroup entry

List cgroup kselftests in the cgroup MAINTAINERS entry.  These are tests
covering core, freezer and cgroup.kill functionality.

Link: https://lkml.kernel.org/r/20220415000133.3955987-4-roman.gushchin@linux.dev
Signed-off-by: Roman Gushchin <roman.gushchin@linux.dev>
Acked-by: Tejun Heo <tj@kernel.org>
Cc: Zefan Li <lizefan.x@bytedance.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Chris Down <chris@chrisdown.name>
Cc: David Vernet <void@manifault.com>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Shakeel Butt <shakeelb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>