Add ability to test UI widgets

From @PeterStaev on April 23, 2016 9:57

From what I see currently it is not possible to test UI widgets. I tried to parse a widget with builder.parse or just create a new object inside the test and set up properties/bindings but both approaches do not initialize the widget fully (i.e. on android the _createUI function is not called, on iOS onLoaded event is never fired). Would be great if we can add tests for UI widgets (especially for people developing UI plugins :))

Copied from original issue: NativeScript/nativescript-unit-test-runner#9

首先调查围绕 builder.parse 和直接创建 widget 的单元测试处理，重点关注报告中提到的 Android _createUI 路径和 iOS onLoaded 事件。确定应如何在测试中执行 widget 生命周期初始化，并在 UI widget 可以被实例化且其平台特定的初始化可以在两个平台上进行测试时，视为工作完成。