zdtm: run test in a new session

It was lost for tests which executed in a new namespaces.
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

test/zdtm/lib/ns.c

@@ -176,6 +176,8 @@ int ns_exec(void *_arg)
 
 	close(args->status_pipe[0]);
 
+	setsid();
+
 	system("ip link set up dev lo");
 
 	ret = dup2(args->status_pipe[1], STATUS_FD);