Build/Test Tools: Declare two TestCase classes as abstract.

TestCases which are intended to be extended and not run directly, should be `abstract`.

Follow-up to [763/tests], [30277].

Props jrf.
See #53363.

git-svn-id: https://develop.svn.wordpress.org/trunk@51585 602fd350-edb4-49c9-b593-d223f7449a82

Author: SergeyBiryukov

tests/phpunit/includes/testcase-canonical.php

@@ -1,6 +1,6 @@
 <?php
 
-class WP_Canonical_UnitTestCase extends WP_UnitTestCase {
+abstract class WP_Canonical_UnitTestCase extends WP_UnitTestCase {
 	public static $old_current_user;
 	public static $author_id;
 	public static $post_ids    = array();

tests/phpunit/includes/testcase-xmlrpc.php

@@ -3,7 +3,7 @@
 require_once ABSPATH . WPINC . '/class-IXR.php';
 require_once ABSPATH . WPINC . '/class-wp-xmlrpc-server.php';
 
-class WP_XMLRPC_UnitTestCase extends WP_UnitTestCase {
+abstract class WP_XMLRPC_UnitTestCase extends WP_UnitTestCase {
 	protected $myxmlrpcserver;
 
 	function set_up() {