[2.1.x] Updated a test example to use snake case.

Backport of ae387776 from master

docs/topics/testing/tools.txt

@@ -1072,7 +1072,7 @@ subclass::
             # Test definitions as before.
             call_setup_methods()
 
-        def testFluffyAnimals(self):
+        def test_fluffy_animals(self):
             # A test that uses the fixtures.
             call_some_test_code()