Add new test case for add() (gh-training#40)

It turns out that we need test coverage for 0+0+0=0

Don't ask me why please

src/calc_test.py

@@ -10,6 +10,9 @@ def test_add_2arg(self):
     def test_add_3arg(self):
         self.assertEqual(add(3, 5, 2), 10, 'adding three, five and two')
 
+    def test_add_zeroes(self):
+        self.assertEqual(add(0, 0, 0), 0 , 'adding zeroes together is zero')
+
     def test_sub_2arg(self):
         # Make sure 4 - 3 = 1
         self.assertEqual(sub(4, 3), 1, 'subtracting three from four')