message: enable mpi solver test with 1 process
diff: @@ -330,7 +330,7 @@ protected:
  }
  {
  SCOPED_TRACE("Some empty partition");
  - guarded_fn(gen_part(50, comm.size() - 1));
  + guarded_fn(gen_part(50, std::max(1, comm.size() - 1)));
  }
  }
  