Tests for Acquisition in Discussion Items

I've been struggling with Acquisition in Discussion Items recently, and i believe the current setup in plone.app.discussion has some faults. I've added a new tests file (test_acquisition.py) which shows these faults (methods test_acquisition_chain and test_view_permission).

I've no idea how to fix them yet, i just wanted to make sure what the problem is first.

Relates to https://dev.plone.org/ticket/13188