mm/damon/tests/core-kunit: split out damos_test_commit_filter() core logic

mm/damon/tests/core-kunit: split out damos_test_commit_filter() core logic

damos_test_commit_filter() is written for only a single test case.  Split
out the core logic of damos_test_commit_filter() as a general one so that
it can be reused for multiple test cases.

Link: https://lkml.kernel.org/r/20251111184415.141757-3-sj@kernel.org
Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: Brendan Higgins <brendan.higgins@linux.dev>
Cc: David Gow <davidgow@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>