Adding Change Node tests

Tests to cover the Change Node. I didn't feel an urge to factor out the common helper.load() function this time as it's not quite as repetitive as in the case of the Switch Node. Please let me know if I shall add utility methods. I personally don't think it's necessary here.
I would personally leave the try/catch block there as otherwise upon a failure, the reported error would be a timeout (done not called!), rather than the actual message coming from should.js. I tested this by deliberately failing one of the tests and see what the behaviour is.

I will of course change the should call to be more straightforward. Thanks