Refactor dependency on wbmodel specific code for testing

[labra]

The code in https://github.com/weso/pschema/tree/master/src/test/scala/es/weso/wdsub/spark/wbmodel is duplicated in sparkwdsub. We should think where we put that code to avoid those duplications.

The problem is that the code depends on both WShEx and graphX so it is a bit tangled with both and creating a different repo for it seems unnecessary.