errc := make(chan error, 1)
	startwritec := make(chan struct{})
	cst := newClientServerTest(t, mode, HandlerFunc(func(w ResponseWriter, r *Request) {
		ctl := NewResponseController(w)
		w.WriteHeader(200)
		if err := ctl.Flush(); err != nil {
			t.Errorf("ctl.Flush() = %v, want nil", err)
		}
		<-startwritec // don't set the deadline until the client reads response headers