efl_ui_table: hardening of pack_at function

efl_ui_table: hardening of pack_at function

_pack_at is used to add new subobjects to the table. When a object is
already part of this table, then we should not add it again. Additional,
when there is already a gi structure, but the parent is something else,
then we should NOT just reuse this struct, otherwise we might use a
struct reference that we do not own. The struct could be owned by
another table widget.

The test must be adjusted, before we did not error on adding a widget
twice. Now we do (just like in box). Hence we should not do that in
test.

Reviewed-by: YeongJong Lee <yj34.lee@samsung.com>
Differential Revision: https://phab.enlightenment.org/D8554