test_exterior_stokes fails on pocl 0.14

With pocl 0.14 the test

PYOPENCL_TEST=port python test_stokes.py "test_exterior_stokes_2d(cl._csc)"

fails with this error:

IT       10 10.0555
Traceback (most recent call last):
  File "test_stokes.py", line 291, in <module>
    exec(sys.argv[1])
  File "<string>", line 1, in <module>
  File "test_stokes.py", line 278, in test_exterior_stokes_2d
    h_max, l2_err = run_exterior_stokes_2d(ctx_factory, nelements)
  File "test_stokes.py", line 161, in run_exterior_stokes_2d
    hard_failure=True)
  File "/home/matt/src/pytential/pytential/solve.py", line 343, in gmres
    stall_iterations=stall_iterations, callback=callback)
  File "/home/matt/src/pytential/pytential/solve.py", line 203, in _gmres
    raise GMRESError(state)
pytential.solve.GMRESError: non-monotonic residuals