seccomp: test, seccomp_filter_threads -- Use multiple threads

Andrew proposed the test which actually triggered the issue
in current seccomp series, put it into a regular basis.

Suggested-by: Andrey Vagin <avagin@virtuozzo.com>
Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>