Add stokes 3D test

test/stokes/stokes_3d_quadrilateral.mini

+__name = stokes_3d_quadrilateral_{__exec_suffix}
+
+__exec_suffix = {diff_suffix}
+diff_suffix = numdiff, symdiff | expand num
+
+cells = 4 4 4
+extension = 1. 1. 1.
+
+[wrapper.vtkcompare]
+name = {__name}
+extension = vtu
+
+[formcompiler]
+numerical_jacobian = 1, 0 | expand num
+exact_solution_expression = g
+compare_l2errorsquared = 1e-10

test/stokes/stokes_3d_quadrilateral.ufl

+cell = hexahedron
+
+x = SpatialCoordinate(cell)
+v_bctype = conditional(x[0] < 1. - 1e-8, 1, 0)
+g_v = as_vector((4.*x[1]*(1.-x[1]), 0.0, 0.0))
+g_p = 8.*(1.-x[0])
+g = (g_v, g_p)
+
+P2 = VectorElement("Lagrange", cell, 2, dirichlet_constraints=v_bctype, dirichlet_expression=g_v)
+P1 = FiniteElement("Lagrange", cell, 1)
+TH = P2 * P1
+
+v, q = TestFunctions(TH)
+u, p = TrialFunctions(TH)
+
+r = (inner(grad(v), grad(u)) - div(v)*p - q*div(u))*dx
+
+forms = [r]