mm, memcg: cg2 memory{.swap,}.peak write tests

Extend two existing tests to cover extracting memory usage through the
newly mutable memory.peak and memory.swap.peak handlers.

In particular, make sure to exercise adding and removing watchers with
overlapping lifetimes so the less-trivial logic gets tested.

The new/updated tests attempt to detect a lack of the write handler by
fstat'ing the memory.peak and memory.swap.peak files and skip the tests if
that's the case.  Additionally, skip if the file doesn't exist at all.

[davidf@vimeo.com: update tests]
  Link: https://lkml.kernel.org/r/20240730231304.761942-3-davidf@vimeo.com
Link: https://lkml.kernel.org/r/20240729143743.34236-3-davidf@vimeo.com


Signed-off-by: David Finkel <davidf@vimeo.com>
Acked-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Michal Koutný <mkoutny@suse.com>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Waiman Long <longman@redhat.com>
Cc: Zefan Li <lizefan.x@bytedance.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>