Two more tests for Sentence model.

These are the result of tracking down a bug, yet to be found.
Tests for the add() function of the Link model.
Replace useless bindModel with ClassRegistry::init

Calling bindModel() makes no sense here, so I assume it was just a way
to access the Contribution model. This brings SQL problems while running
tests because Cake produces invalid querys. ClassRegistry::init() is a
proper way to access an unrelated model.