Add assertion that a test object is not added session
Add assertion that a test object is not added session

This helped me with figuring out whether the `cascade_backrefs=False`
parameter on the backref did the correct thing, and may as well stay
in the test.

Refs #471

agdsn/pycroft,agdsn/pycroft,agdsn/pycroft,agdsn/pycroft,agdsn/pycroft