Tests: reduce race in cache revalidate test.

proxy_cache_revalidate.t

diff --git a/proxy_cache_revalidate.t b/proxy_cache_revalidate.t
index 938e0f2..12a2877 100644
--- a/proxy_cache_revalidate.t
+++ b/proxy_cache_revalidate.t
@@ -93,9 +93,11 @@
 # (a 304 status code will appear in backend's logs), then cached again
 
 like(http_get('/t'), qr/X-Cache-Status: REVALIDATED.*SEE/ms, 'revalidated');
-like(read_file($t->testdir() . '/access.log'), qr/ 304 /, 'not modified');
 like(http_get('/t'), qr/X-Cache-Status: HIT.*SEE/ms, 'request cached');
 
+select undef, undef, undef, 0.1;
+like(read_file($t->testdir() . '/access.log'), qr/ 304 /, 'not modified');
+
 # 2nd document is recreated with a new content
 
 $t->write_file('t2', 'NEW');