selftests/damon/_damon_sysfs: implement kdamonds start function

Extend the tests-writing-purpose DAMON sysfs control module to support the
kdamonds start functionality.

Link: https://lkml.kernel.org/r/20231212194810.54457-3-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>