selftests/damon/sysfs: add obsolete_target test

selftests/damon/sysfs: add obsolete_target test

A new DAMON sysfs file for pin-point target removal, namely
obsolete_target, has been added.  Add a test for the functionality.  It
starts DAMON with three monitoring target processes, mark one in the
middle as obsolete, commit it, and confirm the internal DAMON status is
updated to remove the target in the middle.

Link: https://lkml.kernel.org/r/20251023012535.69625-10-sj@kernel.org
Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: Bijan Tabatabai <bijan311@gmail.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>