selftests/damon/_damon_sysfs: skip testcases if CONFIG_DAMON_SYSFS is disabled

When CONFIG_DAMON_SYSFS is disabled, the selftests fail with the following
outputs,

not ok 2 selftests: damon: sysfs_update_schemes_tried_regions_wss_estimation.py # exit=1
not ok 3 selftests: damon: damos_quota.py # exit=1
not ok 4 selftests: damon: damos_quota_goal.py # exit=1
not ok 5 selftests: damon: damos_apply_interval.py # exit=1
not ok 6 selftests: damon: damos_tried_regions.py # exit=1
not ok 7 selftests: damon: damon_nr_regions.py # exit=1
not ok 11 selftests: damon: sysfs_update_schemes_tried_regions_hang.py # exit=1

The root cause of this issue is that all the testcases above do not check
the sysfs interface of DAMON whether it exists or not.  With this patch
applied, all the testcases above now pass successfully.

Link: https://lkml.kernel.org/r/20250531093937.1555159-1-lienze@kylinos.cn


Signed-off-by: Enze Li <lienze@kylinos.cn>
Reviewed-by: SeongJae Park <sj@kernel.org>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>