Added subtract test

Subtract test was missing

Author: Che931

calculator/calculator_test.py

@@ -8,5 +8,10 @@ def test_add(self):
         result = add(2,2)
         self.assertEqual(result , 4)
 
+    def test_subtract(self):
+        result = subtract(5,3)
+        self.assertEqual(result,2)
+
+
 if __name__ == '__main__':
     unittest.main()