[Tizen] Add unit test for app-widget configuration element handler

This patch adds tizen_appwidget_handler_unittest.cc. New tests verify
that the handler properly handles the following configuration elements:

widget.app-widget @id @primary @update-period @auto-launch
widget.app-widget.box-label @lang #text
widget.app-widget.box-icon @src
widget.app-widget.box-content @src @mouse-event @touch-effect
widget.app-widget.box-content.box-size @preview @use-decoration #text
widget.app-widget.box-content.pd @src @width @height

BUG=XWALK-1515, XWALK-1516
this works too, but = std::string() is a better way