I have removed output for the reduction, but I now get differences above tolerance in the Hamiltonian constraint:

   H.x.asc: substantial differences

      significant differences on 6 (out of 46) lines

      maximum absolute difference in column 13 is 3.46657468355827e-12

      maximum relative difference in column 13 is 9.69181535413736e-11

      (insignificant differences on 9 lines)

   H.y.asc: substantial differences

      significant differences on 7 (out of 46) lines

      maximum absolute difference in column 13 is 2.2119042708546e-12

      maximum relative difference in column 13 is 4.60112943759897e-11

      (insignificant differences on 8 lines)

   H.z.asc: substantial differences

      significant differences on 14 (out of 60) lines

      maximum absolute difference in column 13 is 3.00423574906006e-12

      maximum relative difference in column 13 is 6.2493109449911e-11

      (insignificant differences on 7 lines)

The evolved variables show differences below tolerance.

I attach the patch which contains the testsuites.  I am inclined to commit it, and warn people to use OMP_NUM_THREADS=1 when running it.  At least this gives us a direct working regression test for McLachlan.  But we should understand why the Hamiltonian constraint is giving these differences.

If we want to include this test in the release, we will need to modify the einsteintoolkit.th thornlist to include the new thorn.  The test runs only on 2 processes, as specified in the test.ccl file, due to the CarpetIOASCII output being dependent on the process decomposition.  I have run the test successfully on my laptop and on damiana, but not on anything more exotic.