Added tests for upcoming van Emde Boas tree implementation.

Added to DataStructures/tests.py following standard style.
Added van Emde Boas Tree (vEB Tree) implementation: DataStructures/vEB_tree.py

A van Emde Boas Tree is an efficient priority queue implementation for when the key universe size is known.
Renamed vEB_tree.py -> veb_tree.py to conform to Python naming standards.

Also updated DataStructures/tests.py to reflect this.