The unit test does not really pass for now (it never did before, because it is not finished yet and used for testing the different transfer function behavior only)
This revealed a bug: Transferring single arrows to the surrounding with the "transfer all button" leaves single arrows in the environment, instead of actual stacks.
Transferring them with single clicks stacks the arrows though.
The trick was that the Clonk does not exit the object right away, but gets a drop command instead. Furthermore, it was confusing that the Collection2-callback never actually happens in this situation.