In commit cd4fc15a (Rewrite the testing macros, 2014-03-14)
the vtk_test_mpi_executable() helper was introduced to add
MPI tests.  It depends on the vtk_mpi_link() helper in the
vtkMPI module.  Include the latter in all test CMakeLists.txt
files that add MPI tests.

Change-Id: Ifecc7c6b1bf856a8f66905bfb2d69e22b5de3aac