input: Write a function to design a class that handles an integer list, allowing adding integers to the list and finding the minimum value present in the list. Implement the class and add test cases to verify its functionality.
  assert IntList().find_min() is None
  int_list = IntList()
  int_list.add_value(10)
  int_list.add_value(5)
  int_list.add_value(8)
  assert int_list.find_min() == 5
output: class IntList:
      def __init__(self):
          self.data = []
  
      def add_value(self, value):
          self.data.append(value)
  
      def find_min(self):
          if not self.data:
              return None
          return min(self.data)