mm/damon/core-test: add a kunit test case for ops registration

This commit adds a simple kunit test case for DAMON operations
registration feature.

Link: https://lkml.kernel.org/r/20220419122225.290518-1-sj@kernel.org


Signed-off-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>