message: make mpi session globally available in unit tests
diff: #include "mfem.hpp"
  #include "catch.hpp"
  +mfem::MPI_Session *GlobalMPISession;
  +
  int main(int argc, char *argv[])
  {
  // There must be exactly one instance.
  @@ -27,6 +29,8 @@ int main(int argc, char *argv[])
  #ifdef MFEM_USE_MPI
  mfem::MPI_Session mpi;
  + GlobalMPISession = &mpi;
  +
  // Force tests not tagged as [Parallel] to run only on MPI rank 0
  if (mpi.WorldRank() > 0)
  {
  