mm/damon/tests/core-kunit: add a test case for region merge size limit in damon_test_...

mm/damon/tests/core-kunit: add a test case for region merge size limit in damon_test_merge_regions_of()

Add a test case in damon_test_merge_regions_of() to verify that two
adjacent regions are not merged if the resulting region would exceed the
specified size limit.

Link: https://lkml.kernel.org/r/20251224042200.2061847-4-shu17az@gmail.com
Signed-off-by: Shu Anzai <shu17az@gmail.com>
Reviewed-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>