mm/damon/core-test: test damon_split_region_at()'s access rate copying

mm/damon/core-test: test damon_split_region_at()'s access rate copying

damon_split_region_at() should set access rate related fields of the
resulting regions same.  It may forgotten, and actually there was the
mistake before.  Test it with the unit test case for the function.

Link: https://lkml.kernel.org/r/20231119171529.66863-2-sj@kernel.org
Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: Brendan Higgins <brendanhiggins@google.com>
Cc: David Gow <davidgow@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>