The tests cover new/remove api for nodes and nodetrees and some operators of creating/deleting/copying nodes/groups/materials.
The tests check availability of affected nodes/trees, methods init, copy, free being called, and resulting user counters for trees.
Operation 'make single user' is not covered, because it's not accessible from python.
Calling to 'update' is not checked, because it's purpose is not quite clear.
Tests are modeled for 2 kinds of nodes: simple node just creates nodetree in init and do not care about it further, complex node creates/duplicates/deletes its nodetree.