unittestでDB更新結果のassertする場合、reloadを忘れずに ref: http://qiita.com/nekogeruge_987/items/de0317203cfe59558c4e

lists_controller.rb

def stock
    return redirect_to '/404.html' unless request.xhr?
    @list = List.find(params[:id])

    if @list.is_stocked?
      @list.is_stocked = false
    else
      @list.is_stocked = true
    end

    respond_to do |format|
      if @list.save
        format.js { @list }
      else
        format.js { render :action => "alert/error" }
      end
    end
  end

lists_controller_test.rb

test "is_stockedを更新する" do
    xhr :get, :stock, :id => @stock_flag_is_true.id
    assert_equal false, @stock_flag_is_true.reload.is_stocked
    assert_response :success
end