Fix bad normals in contour test

The contour worklet test was experiencing normals that were set to NaN due to problems with computing the gradient of the original field. It was determined that the problem was caused by the clip filter creating degenerate cells because the clip plane exactly intersected the points of the mesh. Although a problem, we found that this behavior also exists in existing tools like ParaView and VisIt. Thus, it sounds like a problem to be pushed off to a later day. Instead, just move the plane a little bit to get it into general position.