Python - No entiendo los tests en python. Ayuda!

def add_numbers(a, b):

    if a < 0:

        raise ValueError(f'Sorry, no numbers below zero. a={a} and b={b}')

    return a + b

import unittest

from my_math import add_numbers

class TestMyMathFunctions(unittest.TestCase):

    def test_add_numbers_positives(self):

        result = add_numbers(2, 3)

        self.assertEqual(result, 5)

    def test_add_numbers_a_negative(self):

        """Check that an exception is thrown and the error message"""

        with self.assertRaises(ValueError) as error:

            a = -2

            b = 3

            add_numbers(a, b)

        expected_error_message = f"Sorry, no numbers below zero. a={a} and b={b}"

        self.assertEqual(str(error.exception), expected_error_message)

    def test_add_numbers_b_negative(self):

        """Check that an exception is thrown and the error message"""

        with self.assertRaises(ValueError) as error:

            a = 2

            b = -3

            add_numbers(a, b)

        expected_error_message = f"Sorry, no numbers below zero. a={a} and b={b}"

        self.assertEqual(str(error.exception), expected_error_message)

    def test_add_numbers_both_negative(self):

        """Check that an exception is thrown and the error message"""

        with self.assertRaises(ValueError) as error:

            a = -2

            b = -3

            add_numbers(a, b)

        expected_error_message = f"Sorry, no numbers below zero. a={a} and b={b}"

        self.assertEqual(str(error.exception), expected_error_message)

if __name__ == '__main__':

    unittest.main()

FAIL: test_add_numbers_b_negative (__main__.TestMyMathFunctions)

Check that an exception is thrown and the error message

----------------------------------------------------------------------

Traceback (most recent call last):

  File "/tests/test_my_math.py", line 25, in test_add_numbers_b_negative

    add_numbers(a, b)

AssertionError: ValueError not raised

----------------------------------------------------------------------

Ran 4 tests in 0.001s

FAILED (failures=1)

# File: my_math.py

def add_numbers(a, b):

    if a < 0 or b < 0:

        raise ValueError(f'Sorry, no numbers below zero. a={a} and b={b}')

    return a + b

Ran 4 tests in 0.000s

OK