zdtm: Fix race in zdtm/transition/epoll.c test

Child may see close() result before it receives signal,
while it shouldn't see it. Instead of games with later
close(), just stop do it. sys_exit() after program finish
will close them all.

Reported-by: Andrey Vagin <avagin@virtuozzo.com>
Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>