[1.1.X] Fixed #12093 -- `LayerMapping` now takes into account model...

[1.1.X] Fixed #12093 -- `LayerMapping` now takes into account model inheritance when looking for the geometry column; forgot to diable extent regression test on MySQL.

Backport of r11703 from trunk.


git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.1.X@11704 bcc190cf-cafb-0310-a4f2-bffc1f526a37