dirichlet_fem run with updated libMesh/fparser

This also seems to be consistent between computers/partitionings, where the old runs were not.  The regression here stemmed from https://github.com/libMesh/libmesh/commit/c403f3efc9f1a8352c960b6422802ccf9aef2498 but that can't be the cause.

We need to get a more rigorous test of this functionality set up at some point...