MDL-77350 message: Added missing class properties

In PHP 8.2 and later, setting a value to an undeclared class property is
deprecated and emits a deprecation notice.
So we need to add missing class properties that still need to be declared.

message/output/popup/tests/api_test.php

@@ -34,6 +34,9 @@ require_once($CFG->dirroot . '/message/output/popup/tests/base.php');
 class api_test extends \advanced_testcase {
     use \message_popup_test_helper;
 
+    /** @var \phpunit_message_sink message redirection. */
+    public $messagesink;
+
     /**
      * Test set up.
      *

message/output/popup/tests/externallib_test.php

@@ -37,6 +37,9 @@ require_once($CFG->dirroot . '/message/output/popup/tests/base.php');
 class externallib_test extends \advanced_testcase {
     use message_popup_test_helper;
 
+    /** @var \phpunit_message_sink message redirection. */
+    public $messagesink;
+
     /**
      * Test set up.
      *