selftests/damon/sysfs: test DAMOS apply intervals

Update DAMON selftests to test existence of the file for reading/writing
DAMOS apply interval under each scheme directory.

Link: https://lkml.kernel.org/r/20230916020945.47296-8-sj@kernel.orgSigned-off-by: SeongJae Park <sj@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Steven Rostedt (Google) <rostedt@goodmis.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

tools/testing/selftests/damon/sysfs.sh

@@ -175,6 +175,7 @@ test_scheme()
 	ensure_dir "$scheme_dir" "exist"
 	ensure_file "$scheme_dir/action" "exist" "600"
 	test_access_pattern "$scheme_dir/access_pattern"
+	ensure_file "$scheme_dir/apply_interval_us" "exist" "600"
 	test_quotas "$scheme_dir/quotas"
 	test_watermarks "$scheme_dir/watermarks"
 	test_filters "$scheme_dir/filters"