input: Write a python function to test the Stack class.
  assert stack.is_empty() == True
  stack.push(1)
  assert stack.is_empty() == False
  assert stack.peek() == 1
  assert stack.size() == 1
  stack.push(2)
  assert stack.peek() == 2
  assert stack.size() == 2
  assert stack.pop() == 2
  assert stack.peek() == 1
  assert stack.size() == 1
  assert stack.pop() == 1
  assert stack.is_empty() == True
output: def test_stack():
      stack = Stack()
      assert stack.is_empty() == True
      stack.push(1)
      assert stack.is_empty() == False
      assert stack.peek() == 1
      assert stack.size() == 1
      stack.push(2)
      assert stack.peek() == 2
      assert stack.size() == 2
      assert stack.pop() == 2
      assert stack.peek() == 1
      assert stack.size() == 1
      assert stack.pop() == 1
      assert stack.is_empty() == True