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

首先調查圍繞 builder.parse 和直接建立 widget 的單元測試處理，重點關注報告中提到的 Android _createUI 路徑和 iOS onLoaded 事件。確定應如何在測試中執行 widget 生命週期初始化，並在 UI widget 可以被具現化且其平台特定的初始化可以在兩個平台上進行測試時，視為工作完成。