Fixup the nonlinear test

test/nonlinear/nonlinear_dg.ufl

-f = Expression("return -2.0*x.size();")
-g = Expression("return x.two_norm2();", on_intersection=True)
+cell = "triangle"
+x = SpatialCoordinate(cell)
+
+f = -4.
+g = x[0]*x[0] + x[1]*x[1]
 
-cell = triangle
 V = FiniteElement("DG", cell, 1)
 
 u = TrialFunction(V)