When the closest point on a surface to a test point is on an edge, the
area-weighted normal is the average of the normals of the faces
adjacent to the edge. The contribution from one of the faces was being
added twice, producing the incorrect area-weighted normal. This
resulted in the wrong sign for the signed distance in some cases.
This patch fixes the error.

Changed TestImplicitPolyDataDistance.cxx to fail prior to this change
and pass after the change. The previous test exercised
vtkImplicityPolyDataDistance but did not check the results. The new
test checks against a baseline.