zdtm: don't take into account cpt/rst time in posix_timer

Add cpt/rst timeout to inaccuracy.

All timers are stopped during cpt/rst. We don't know in which moment
they are stopped and run, so this timeout is added to acceptable
inaccuracy.

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>