jetpack: Fix test class comment (#42228)

This reference to a Core class got incorrectly changed in #42028.
Further, it should really be either a file comment or a class comment
(#37290 seems to have screwed that up).

Author: anomiex

projects/plugins/jetpack/changelog/fix-comment-referring-to-core-class

@@ -0,0 +1,5 @@
+Significance: patch
+Type: other
+Comment: Fix test class comment
+
+

projects/plugins/jetpack/tests/php/lib/Jetpack_REST_TestCase.php

@@ -1,9 +1,10 @@
 <?php
 /**
+ * This is WP_Test_REST_Controller_Testcase without the unneeded abstract methods.
+ *
  * @package automattic/jetpack
  */
 
-// This is REST_Controller_Testcase without the unneeded abstract methods.
 require_once __DIR__ . '/class-jptest-spy-rest-server.php';
 
 if ( defined( 'IS_WPCOM' ) && IS_WPCOM ) {