Added another test for addition functionality

src/test/py/test_primitive_calculations.py

@@ -12,6 +12,9 @@ class primitive_calculations(unittest.TestCase):
     def test_add_5_plus_5_equals_10(self):
         self.assertEqual(add(5, 5), 10)
 
+    def test_add_minus_5_plus_minus_5_equals_10(self):
+        self.assertEqual(add(-5, -5), -10)
+
 
 if __name__ == "__main__":
     unittest.main()