efl_ui_spec_suite: support custom widgets

efl_ui_spec_suite: support custom widgets

sooo, this test suite is testing interfaces. And when a content needs to
be passed into the API, a lightweight version of a widget will be passed
in. But sometimes implementations *do* requires special classes, they
can now be specified with a custom-mapping in json.

Reviewed-by: SangHyeon Jade Lee <sh10233.lee@samsung.com>
Differential Revision: https://phab.enlightenment.org/D8701