Tests: proxy_cache_use_stale.t speedup.

Tests for not blocked stale responses are adjusted to have a lesser new
response size.  It should be enough to delay sending a new response for
just a couple of seconds to catch the old bug fixed in 1.13.1.  This
allows to speed up shutting down nginx that waits for those responses.

proxy_cache_use_stale.t

diff --git a/proxy_cache_use_stale.t b/proxy_cache_use_stale.t
index 9326636..78eb2de 100644
--- a/proxy_cache_use_stale.t
+++ b/proxy_cache_use_stale.t
@@ -238,12 +238,12 @@
 # before 1.13.1, if stale response was not sent in one pass, its remaining
 # part was blocked and not sent until background update has been finished
 
-$t->write_file('t7.html', 'SEE-THAT' x 1024);
+$t->write_file('t7.html', 'SEE-THAT' x 256);
 
 my $r = read_all(get('/t7.html?lim=1', 'max-age=1', start => 1));
 like($r, qr/STALE.*^(SEE-THIS){1024}$/ms, 's-w-r - stale response not blocked');
 
-$t->write_file('t9.html', 'SEE-THAT' x 1024);
+$t->write_file('t9.html', 'SEE-THAT' x 256);
 $t->write_file('ssi.html', 'xxx <!--#include virtual="/t9.html?lim=1" --> xxx');
 
 $r = read_all(http_get('/ssi.html', start => 1));