Remove expected failure in mixed extruded Helmholtz test

We were checking the wrong component of the solution, hence not seeing
convergence.  Use Schur complement preconditioner to obtain solver
convergence on larger problems.  Edit mesh mesh range to remove exact
solves.
Use more mesh sizes in mixed extruded Helmholtz test
Merge pull request #91 from firedrakeproject/helmholtzfix

Fix mixed extruded Helmholtz test.