mm/damon/tests/core-kunit: add a test for damos_set_filters_default_reject()

DAMOS filters' default reject behavior is not very simple.  Actually there
was a mistake[1] during the development.  Add a kunit test for validating
the behavior.

Link: https://lkml.kernel.org/r/20250513002715.40126-5-sj@kernel.org
Link: https://lore.kernel.org/20250227002913.19359-1-sj@kernel.org

 [1]
Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: Brendan Higgins <brendan.higgins@linux.dev>
Cc: David Gow <davidgow@google.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>