Summary:
This patch remove evas_box function from Efl.Ui.Box_Flow and add unit test.
Depends on D8214
Test Plan:
1. make check
2. `elementary_test -to 'efl.ui.box'` with 'flow' checkbox.
Reviewers: zmike, Jaehyun_Cho
Reviewed By: zmike
Subscribers: cedric, #reviewers, #committers
Tags: #efl
Differential Revision: https://phab.enlightenment.org/D8219