Tests: tune proxy_timeout in stream_udp_limit_rate.t.

When something went wrong, an existing stream session created upon the first
client datagram could be finalized, resulting in tests being painfully slow
due to bad interaction between proxy_requests and high proxy_timeout default.

stream_udp_limit_rate.t

diff --git a/stream_udp_limit_rate.t b/stream_udp_limit_rate.t
index af59a3f..68a15dd 100644
--- a/stream_udp_limit_rate.t
+++ b/stream_udp_limit_rate.t
@@ -36,6 +36,7 @@
 stream {
     proxy_requests           2;
     proxy_responses          1;
+    proxy_timeout            3s;
 
     server {
         listen               127.0.0.1:%%PORT_8982_UDP%% udp;