input: Write a python function to test the PriorityQueue class.
  assert pq.insert(3) == None
  assert pq.insert(1) == None
  assert pq.insert(2) == None
  assert pq.pop() == 3
  assert pq.pop() == 2
  assert pq.pop() == 1
  assert pq.pop() == None
output: class PriorityQueue:
      def __init__(self):
          self.queue = []
  
      def insert(self, item):
          self.queue.append(item)
          self.queue.sort(reverse=True)
  
      def pop(self):
          if not self.queue:
              return None
          return self.queue.pop(0)
  
  def test_PriorityQueue():
      pq = PriorityQueue()
      assert pq.insert(3) == None
      assert pq.insert(1) == None
      assert pq.insert(2) == None
      assert pq.pop() == 3
      assert pq.pop() == 2
      assert pq.pop() == 1
      assert pq.pop() == None