mm/damon/sysfs-test: add a unit test for damon_sysfs_set_targets()

damon_sysfs_set_targets() had a bug that can result in unexpected memory
usage and monitoring overhead increase. The bug has fixed by a previous
commit. Add a unit test for avoiding a similar bug of future.

Link: https://lkml.kernel.org/r/20231022210735.46409-3-sj@kernel.org
Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>