selftests/damon/_damon_sysfs: support DAMOS apply interval

Update the test-purpose DAMON sysfs control Python module to support DAMOS
apply interval.

Link: https://lkml.kernel.org/r/20240207203134.69976-4-sj@kernel.org


Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>