how about:

class TestDivFunction(unittest.TestCase):

    def test_division(self):
        self.assertEqual(div(10, 2), 5)
        self.assertEqual(div(-10, 2), -5)
        self.assertEqual(div(0, 1), 0)

    def test_division_by_zero(self):
        self.assertTrue(math.isnan(div(10, 0)))