IDEAS FOR A TEST PLAN
For example your plan to test the "Height" operation might say
something like, 
Test the following types of trees: 
-  an empty tree,
 -  a tree with one node,
 -  a tree with many nodes in which the longest path
     terminates close to the left side of the tree,
 -  a tree with many nodes in which the longest path
     terminates close to the right side of the tree, and
 -  a tree with many nodes in which the longest path
     terminates close to the middle of the tree.