mm/damon/core-test: add a unit test for damon_moving_sum()

Add a simple unit test for the pseudo moving-sum function
(damon_moving_sum()).

Link: https://lkml.kernel.org/r/20230915025251.72816-5-sj@kernel.org


Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>