Tests: adjusted read timeout in stream_udp_limit_conn.t.

Now that the test is expected to be positive on mainline, increase the timeout.
Note that it should still be below proxy_timeout to not break a subsequent test
on stable branch.
diff --git a/stream_udp_limit_conn.t b/stream_udp_limit_conn.t
index 1e81a3e..bc82a9e 100644
--- a/stream_udp_limit_conn.t
+++ b/stream_udp_limit_conn.t
@@ -93,7 +93,7 @@
 TODO: {
 local $TODO = 'not yet' unless $t->has_version('1.15.0');
 
-is($s->io('1', read_timeout => 0.1), '1', 'passed new request');
+is($s->io('1', read_timeout => 0.4), '1', 'passed new request');
 
 }