Fixed naming typo.

vEB_tree -> veb_tree in DataStructures/tests.py to reflect new name of veb_tree.py
Slightly improved readability and efficiency of Sorting/bubble_sort.

- Bubble Sort now keeps track of which part of the list is already sorted (the end) and stops subsequent passes before it.
- Variable name changes for improved clarity.
Updated Sorting/counting_sort.py

Refactored Sorting/counting_sort.py for better clarity. List traversal when updating values in data is now done forwards rather than backwards.
Improved clarity of Sorting/heap_sort.py
Slight comment update in Sorting/merge_sort.py and Sorting/quicksort.py
Slight update to looping bounds in Sorting/selection_sort.py

selection_sort.py was doing one more outer loop iteration than it had to.
Updated rand_list in Sorting/tests.py.
Updated Sorting/tests.py.

More usage of rand_list() for better readability, and checking two-value input cases is now done in a more efficient way.