Tests: adjusted proxy_cache_use_stale.t for slow hosts.

The request processing which updates proxy cache could be delayed for too
little time before a subsequent request would be started processing, such
that the resulting cache status will be HIT instead of expected UPDATING.

proxy_cache_use_stale.t

diff --git a/proxy_cache_use_stale.t b/proxy_cache_use_stale.t
index 6559e98..88da87b 100644
--- a/proxy_cache_use_stale.t
+++ b/proxy_cache_use_stale.t
@@ -39,7 +39,7 @@
 
     proxy_cache_path   %%TESTDIR%%/cache  levels=1:2  keys_zone=NAME:1m;
 
-    limit_req_zone  $binary_remote_addr  zone=one:1m  rate=15r/m;
+    limit_req_zone  $binary_remote_addr  zone=one:1m  rate=10r/m;
 
     server {
         listen       127.0.0.1:8080;