Use both symbolic/non-symbolic in failing poisson_dg 3D test

test/sumfact/poisson/poisson_dg_3d.ufl

 cell = "hexahedron"
 
-x = SpatialCoordinate(cell)
-c = (0.5-x[0])**2 + (0.5-x[1])**2 + (0.5-x[2])**2
-g = exp(-1.*c)
-f = 6*(1.-c)*g
+#x = SpatialCoordinate(cell)
+#c = (0.5-x[0])**2 + (0.5-x[1])**2 + (0.5-x[2])**2
+#g = exp(-1.*c)
+#f = 6*(1.-c)*g
+
+f = Expression("Dune::FieldVector<double,2> c(0.5); c-= x; return 4.*(1.-c.two_norm2())*std::exp(-1.*c.two_norm2());", cell=cell)
+g = Expression("Dune::FieldVector<double,2> c(0.5); c-= x; return std::exp(-1.*c.two_norm2());", on_intersection=True, cell=cell)
 
 V = FiniteElement("DG", cell, degree)