Added another test for subtraction functionality

src/test/py/test_primitive_calculations.py

@@ -24,6 +24,9 @@ class primitive_calculations(unittest.TestCase):
     def test_subtract_5_minus_5_equals_0(self):
         self.assertEqual(subract(5, 5), 0)
 
+    def test_subtract_minus_5_minus_minus_5_equals_0(self):
+        self.assertEqual(subract(-5, -5), 0)
+
 
 if __name__ == "__main__":
     unittest.main()