net/http: increase timeout length for TestOnlyWriteTimeout

This test was sometimes timing out on the plan9/arm builder
(raspberry pi) when run in parallel with other network intensive
tests. It appears that tcp on the loopback interface could do
with some tuning for better performance on Plan 9, but until
that's done, increasing the timeout from 5 to 10 seconds allows
this test to pass.  This should have no effect on other platforms
where 5 seconds was already enough.

Change-Id: If310ee569cae8ca8f56346d84ce23803feb23a41
Reviewed-on: https://go-review.googlesource.com/94796Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/http/serve_test.go

@@ -931,7 +931,7 @@ func TestOnlyWriteTimeout(t *testing.T) {
 		if err == nil {
 			t.Errorf("expected an error from Get request")
 		}
-	case <-time.After(5 * time.Second):
+	case <-time.After(10 * time.Second):
 		t.Fatal("timeout waiting for Get error")
 	}
 	if err := <-afterTimeoutErrc; err == nil {