efl_ui_radio_group: migrate to Efl.Ui.Single_Selectable
with this the whole thing is migrated to single_selectable. The group is
added to the spec test suite. The elm_test case of radio_group now also has
a fallback option to demonstrate the usage of it.
This also fixes a broken testcase, where a flag was forgotten to be
Reviewed-by: Mike Blumenkrantz <firstname.lastname@example.org>
Differential Revision: https://phab.enlightenment.org/D9706