new_contents: import unittest
  from rawdisk.util.singleton import Singleton
  
  class MockSingleton(metaclass=Singleton):
      def __init__(self):
          self.x = 0x10
  
  class TestSingleton(unittest.TestCase):
      def testReturnsSameInstance(self):
          first = MockSingleton()
          second = MockSingleton()
  
          self.assertIs(first, second)
  
subject: add singleton test