selftests: cgroup: add zswap-memcg unwanted writeback test

selftests: cgroup: add zswap-memcg unwanted writeback test

Add a test to verify that when a memcg hits its limit in zswap, it doesn't
trigger an unwanted writeback that would result in pages not owned by that
memcg to be sent to disk, even if zswap isn't full.  This was fixed by
commit 0bdf0efa180a("zswap: do not shrink if cgroup may not zswap").

Link: https://lkml.kernel.org/r/20230621153548.428093-4-cerasuolodomenico@gmail.com
Signed-off-by: Domenico Cerasuolo <cerasuolodomenico@gmail.com>
Cc: Dan Streetman <ddstreet@ieee.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Nhat Pham <nphamcs@gmail.com>
Cc: Rik van Riel <riel@surriel.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Seth Jennings <sjenning@redhat.com>
Cc: Shakeel Butt <shakeelb@google.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Tejun Heo <tj@kernel.org>
Cc: Vitaly Wool <vitaly.wool@konsulko.com>
Cc: Zefan Li <lizefan.x@bytedance.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>