Update outdated class name in "How to Write a Checker" test example (#5449)

* Update code example in "How to Write a Checker"

`pylint.testutils.Message` got first renamed to
`pylint.testutils.TestMessage` in 49776ac and finally renamed to
`pylint.testutils.MessageTest` in 24cbf8c.

* Add pylbrecht to contributors

Co-authored-by: Philipp Albrecht <[email protected]>
Co-authored-by: Pierre Sassoulas <[email protected]>

Author: 3 people

CONTRIBUTORS.txt

@@ -583,5 +583,7 @@ contributors:
 
 * Felix von Drigalski (felixvd): contributor
 
+* Philipp Albrecht (pylbrecht): contributor
+
 * Allan Chandler (allanc65): contributor
   - Fixed issue 5452, false positive missing-param-doc for multi-line Google-style params

doc/how_tos/custom_checkers.rst

@@ -271,7 +271,7 @@ We can use the example code that we used for debugging as our test cases.
           self.checker.visit_functiondef(func_node)
           self.checker.visit_return(return_node_a)
           with self.assertAddsMessages(
-              pylint.testutils.Message(
+              pylint.testutils.MessageTest(
                   msg_id='non-unique-returns',
                   node=return_node_b,
               ),