update testSaveTransaction as it was not relevant

Validation occurs before the event is sent.
As the validation failed, event was not sent. So test was not relevant.
solution :
- use the global Event Manager.
- try to save a correct 'Post' data.
- the first time the callback is called, try to save a correct 'Article' data.
- the second time the callback is called, stop the event to rollback everything.