input: Write a function to implement a data structure in Python that stores integer values and provides methods to add a value and find the minimum value in the stored values. The function should include a class named `IntList` with an `add_value` method to add values and a `find_min` method to return the minimum value. Implement the class and add necessary assertions to test the functionality.
  
  assert IntList().find_min() == None
  assert IntList().add_value(3).find_min() == 3
  assert IntList().add_value(4).add_value(2).add_value(1).find_min() == 1
output: class IntList:
      def __init__(self):
          self.values = []
  
      def add_value(self, value):
          self.values.append(value)
          return self
  
      def find_min(self):
          if not self.values:
              return None
          return min(self.values)