improve test for fr.xebia.cascading.learn.level4.NonLinearDataflow.split

[BertrandDechoux]

People manage to 'cheat' by bidding three different pipes to the source. 0) Check whether there really is a difference in the dataflow

1. Find out if there is a way to introspect the dataflow to check that there is only one pipe associated with the source
2. If not, maybe change the 'api' in order to discourage this kind of solution