Make test for ST_PointOnSurface accept any contained point as valid

This is to support a change in GEOS 3.3.9 / 3.4.0 that started
giving different (but still valid) results.
See http://trac.osgeo.org/geos/ticket/623

git-svn-id: http://svn.osgeo.org/postgis/branches/2.0@11161 b70326c6-7e19-0410-871a-916f4a2858ee