Fix sinon.stub deprecation warnings

Calls to `sinon.stub(obj, 'meth', fn)` are deprecated and therefore
replaced by `sinon.stub(obj, 'meth).callsFake(fn)` as instructed by
the deprecation warning.

This makes the js unit testing output readable again.

Signed-off-by: Christoph Wurst <christoph@winzerhof-wurst.at>