Add unit tests for derived class volume() and true_centroid() implementations.
Add unit tests for derived class volume() and true_centroid() implementations.

Refs #2978

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