net/http/cgi: increase a flaky test timeout

Fixes 2450, probably.

R=golang-dev, gri
CC=golang-dev
https://golang.org/cl/5540074

src/pkg/net/http/cgi/host_test.go

@@ -364,7 +364,7 @@ func TestCopyError(t *testing.T) {
 	conn.Close()
 
 	tries := 0
-	for tries < 15 && childRunning() {
+	for tries < 25 && childRunning() {
 		time.Sleep(50 * time.Millisecond * time.Duration(tries))
 		tries++
 	}