tests: mpi-fix for GTest

each MPI task should return the same test result; otherwise it might
happen that the test passed on rank 0 but one of the minor ranks finally
wins the race on returning from main

Author: torbjoernk

tests/examples/advection_diffusion/test_mpi_advection_diffusion.cpp

@@ -51,6 +51,7 @@ int main(int argc, char** argv)
   if (rank == 0) {
     result = RUN_ALL_TESTS();
   }
+  MPI_Bcast(&result, 1, MPI_INT, 0, MPI_COMM_WORLD);
   MPI_Finalize();
   return result;
 }