added test to use sqrt_power with negative number as exponent

src/test/py/test_calculations_with_roots.py

@@ -30,5 +30,7 @@ class calculationsWithRoots(unittest.TestCase):
     def test_sqrt_16_to_power_of_0_should_be_1(self):
         self.assertEqual(sqrt_power(16, 0), 1)
 
+    def test_sqrt_4_to_power_of_negative_3_should_be_one_eighth(self):
+        self.assertEqual(sqrt_power(4, -3), 0.125)
 if __name__ == '__main__':
     unittest.main()