Added tests for linked list implementation.

Tests for upcoming linked list implementation added to DataStructures/tests.py
Added linked list implementation: DataStructures/LinkedList

Although a linked list itself is not very useful in Python, this shows a possible implementation if one is required, based off of the Java.util.List interface.
Resolved ambiguity issues in DataStrucutres/tests.py

- Updated DataStructures/queue.py -> DataStructures/dequeue.py, stops naming conflict with built in Queue
- Made class references in DataStructures/tests.py more explicit by referencing through module names explicitly