Reworked PriorityQueue and Added Tests (#1025)

* Reworked PriorityQueue spec

Modified:
- Priority Queue methods:
    queue[elem] now returns the first value of elem stored in queue
    elem in queue now correctly returns whether a copy of element is present regardless of the function value. Apparently the bug was introduced while trying to meet heapq spec
    del queue[elem] deletes the first instance of elem in queue correctly
- Algorithms
    Same change in best_first_graph_search in romania_problem.py and search.py to make them compatible with the new spec
- Tests
    Introduced 3 tests in test_utils.py to comprehensively test PriorityQueue's new spec

* Reworked PriorityQueue spec

Modified:
- Priority Queue methods:
    queue[elem] now returns the first value of elem stored in queue
    elem in queue now correctly returns whether a copy of element is present regardless of the function value. Apparently the bug was introduced while trying to meet heapq spec
    del queue[elem] deletes the first instance of elem in queue correctly
- Algorithms
    Same change in best_first_graph_search in romania_problem.py and search.py to make them compatible with the new spec
- Tests
    Introduced 3 tests in test_utils.py to comprehensively test PriorityQueue's new spec