selftests/damon/_damon_sysfs: support obsolete_target file

selftests/damon/_damon_sysfs: support obsolete_target file

A DAMON sysfs file, namely obsolete_target, has been newly introduced.
Add a support of that file to _damon_sysfs.py so that DAMON selftests for
the file can be easily written.

Link: https://lkml.kernel.org/r/20251023012535.69625-7-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>