2D only fixes

These are fixes to allow the case when libMesh was configured in --enable-2D-only mode. Note I had to add a new m4 to test for LIBMESH_DIM so we could skip a couple of tests that needed 3D. Actually, I'm not sure that redistribute_regression needs 3D (looks like it's using a 2D mesh?), but the parsed function has a {z} component that breaks in 2D only mode, so just skipping it for now.

I did not try to deal with the --enable-1D-only case as 1. I don't need it right now and 2. That will be a much larger endeavor, I think.