WIP zipper: major change, adding immutability tests and node insertion tests

[cglacet]

The main goal is to be able to run tests without using an intermediate tree structure.

This follows issue exercism/python#1497.

Now the test consists in creating a tree starting from an empty zipper using the the atomic operations left, right, up and insert/modify/read node values. (Each operation is tested independently.)

In the old version there were no immutability check. Immutability will now be an important part of students goals.