projection unit tests: use subdomain-specific vars
projection unit tests: use subdomain-specific vars

This should fail on some partitionings in a similar way to #1919 did,
and so should protect us a little from regressions in the
subdomain-restricted-variable support in GenericProjector

BalticPinguin/libmesh,pbauman/libmesh,BalticPinguin/libmesh,capitalaslash/libmesh,pbauman/libmesh,roystgnr/libmesh,90jrong/libmesh,libMesh/libmesh,libMesh/libmesh,jwpeterson/libmesh,hrittich/libmesh,hrittich/libmesh,pbauman/libmesh,90jrong/libmesh,BalticPinguin/libmesh,roystgnr/libmesh,BalticPinguin/libmesh,roystgnr/libmesh,jwpeterson/libmesh,capitalaslash/libmesh,hrittich/libmesh,hrittich/libmesh,pbauman/libmesh,libMesh/libmesh,dschwen/libmesh,90jrong/libmesh,hrittich/libmesh,libMesh/libmesh,90jrong/libmesh,dschwen/libmesh,BalticPinguin/libmesh,libMesh/libmesh,pbauman/libmesh,libMesh/libmesh,BalticPinguin/libmesh,roystgnr/libmesh,hrittich/libmesh,BalticPinguin/libmesh,90jrong/libmesh,roystgnr/libmesh,capitalaslash/libmesh,libMesh/libmesh,hrittich/libmesh,dschwen/libmesh,90jrong/libmesh,pbauman/libmesh,roystgnr/libmesh,jwpeterson/libmesh,hrittich/libmesh,libMesh/libmesh,hrittich/libmesh,roystgnr/libmesh,capitalaslash/libmesh,jwpeterson/libmesh,dschwen/libmesh,capitalaslash/libmesh,90jrong/libmesh,dschwen/libmesh,pbauman/libmesh,capitalaslash/libmesh,pbauman/libmesh,roystgnr/libmesh,jwpeterson/libmesh,jwpeterson/libmesh,capitalaslash/libmesh,90jrong/libmesh,dschwen/libmesh,BalticPinguin/libmesh,pbauman/libmesh,capitalaslash/libmesh,jwpeterson/libmesh,dschwen/libmesh,dschwen/libmesh,90jrong/libmesh,jwpeterson/libmesh