Drop new test Node field introduced in 8ce2223b6952567b28adfcc11c2

8ce2223b6952567b28adfcc11c23c9f19c4c0324 introduced a new field in
the "Node" objects used in our functional tests - the node_id. Its
not a bad idea to cache it, but unless we want to commit and use it
everywhere, we should avoid duplicating data, especially in tests
where we'd rather exercise the underlying code than bypass it.