input: Write a python function to test the add and subtract methods of the Calculator class.
  assert Calculator().add(2, 3) == 5
  assert Calculator().add(-1, 1) == 0
  assert Calculator().subtract(5, 3) == 2
  assert Calculator().subtract(10, 15) == -5
output: class Calculator:
      def add(self, a, b):
          return a + b
  
      def subtract(self, a, b):
          return a - b
  
  def test_calculator():
      calc = Calculator()
      assert calc.add(2, 3) == 5
      assert calc.add(-1, 1) == 0
      assert calc.subtract(5, 3) == 2
      assert calc.subtract(10, 15) == -5