input: Write a function to test the Stack class methods (isEmpty, push, pop, peek, size).
  assert stack.isEmpty() == True
  stack.push(1)
  assert stack.isEmpty() == False
  stack.push(2)
  assert stack.peek() == 2
  assert stack.size() == 2
  assert stack.pop() == 2
  assert stack.size() == 1
  assert stack.peek() == 1
  assert stack.pop() == 1
  assert stack.isEmpty() == True
  assert stack.pop() == "Stack is empty"
  assert stack.peek() == "Stack is empty"
output: class Stack:
      def __init__(self):
          self.items = []
  
      def isEmpty(self):
          return len(self.items) == 0
  
      def push(self, item):
          self.items.append(item)
  
      def pop(self):
          if self.isEmpty():
              return "Stack is empty"
          return self.items.pop()
  
      def peek(self):
          if self.isEmpty():
              return "Stack is empty"
          return self.items[-1]
  
      def size(self):
          return len(self.items)
  
  def test_stack():
      stack = Stack()
      assert stack.isEmpty() == True
      stack.push(1)
      assert stack.isEmpty() == False
      stack.push(2)
      assert stack.peek() == 2
      assert stack.size() == 2
      assert stack.pop() == 2
      assert stack.size() == 1
      assert stack.peek() == 1
      assert stack.pop() == 1
      assert stack.isEmpty() == True
      assert stack.pop() == "Stack is empty"
      assert stack.peek() == "Stack is empty"