Add tests for result return after remove element.
Make enhancement on List_remove function.
Makesure the List_remove now is suitable for different length of Linked List.
Take out any meaningless test, comment any testable printf code.
Create test function to delete an element within 4.
Keep modify List_remove.
Add all test_RM function for any deleted element.
Addition: Tested with 5 elements and it work.
Add test to remove last element when it is isolated.
No remove operation will be done since there are no pointer point
to last element. It should return a NULL address in this case.