Relax inert checks

Previously, when creating an object from inert inputs (eg:
"hb_font_create(hb_face_get_empty())") we returned the inert
empty object.  This is not helpful as there are legitimate
usecases to do that.

We now never return the inert object unless allocation failed.

Tests are revised to reflect.
[utils] Add hb-fc-list

This is a tool that lists all fonts that can render a given string.
It uses hb_shape() to do so, and as such is aware of HarfBuzz's
normalizer.