update testSaveTransaction as still not relevant

update (again) the test due to my stupidity.
- the second time the callback is called, the 'Article' data are still not saved, so there will be no data to rollback.
new test which will hopefully work :
- use the 'Post' specific Event Manager to attach the callback.
- when callback is called, try to save 'Article' data.
- assert Article->save() succeeded.
- stop event and return false to force rollback in Post->save()