test: make write_pidfile atomic

This is useful so that hooks can do a $(cat $pidfile) or [ -f $pidfile ]
and rely on the result.

Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
It-makes-sence-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>