zdtm: setup sigchld handler after starting the test

When a process is started in a new pid namespace, the init process must
colleck zombies.

But before this the init process must waits the root test process, which
returns a status of preparation stage.

Currently these two operation can race, because the both ones call waitpid().

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