Tests: aligned various generic read timeouts to http_end().

It was observed that previous timeout values could not be enough when
making mass-building, in particular, when running with modsecurity.
diff --git a/h2_proxy_request_buffering.t b/h2_proxy_request_buffering.t
index f72cc24..75a22b1 100644
--- a/h2_proxy_request_buffering.t
+++ b/h2_proxy_request_buffering.t
@@ -265,7 +265,7 @@
 	my ($s, $timo) = @_;
 	my $buf = '';
 
-	if (IO::Select->new($s)->can_read($timo || 5)) {
+	if (IO::Select->new($s)->can_read($timo || 8)) {
 		$s->sysread($buf, 16384) or return;
 		log2i($buf);
 	}