'``Losetup.add`` creates a device that is listed.'

def test_success(self):

    loop = Losetup()
    backing_file = self.make_temporary_file()
    with backing_file.open('wb') as f:
        f.truncate(LOOPBACK_MINIMUM_ALLOCATABLE_SIZE)
    device = loop.add(backing_file)
    self.addCleanup(device.remove)
    self.assertIn(device, loop.list())
