Avoid corner-only mesh connection in SlitMesh test
Avoid corner-only mesh connection in SlitMesh test

I've long said we don't support meshes where a manifold is only
connected at one node, but apparently we did kind of support it
accidentally in at least simple cases?  But improving
GhostPointNeighbors performance breaks this case, so let's extend the
mesh to something we've actually committed to handle.

BalticPinguin/libmesh,dschwen/libmesh,libMesh/libmesh,dschwen/libmesh,BalticPinguin/libmesh,dschwen/libmesh,libMesh/libmesh,jwpeterson/libmesh,libMesh/libmesh,jwpeterson/libmesh,roystgnr/libmesh,BalticPinguin/libmesh,dschwen/libmesh,dschwen/libmesh,libMesh/libmesh,libMesh/libmesh,jwpeterson/libmesh,roystgnr/libmesh,BalticPinguin/libmesh,roystgnr/libmesh,libMesh/libmesh,libMesh/libmesh,jwpeterson/libmesh,jwpeterson/libmesh,roystgnr/libmesh,libMesh/libmesh,roystgnr/libmesh,roystgnr/libmesh,jwpeterson/libmesh,BalticPinguin/libmesh,dschwen/libmesh,roystgnr/libmesh,BalticPinguin/libmesh,BalticPinguin/libmesh,roystgnr/libmesh,BalticPinguin/libmesh,dschwen/libmesh,jwpeterson/libmesh,jwpeterson/libmesh,dschwen/libmesh