Tests: Change export_wp filter callback to static function

Author: himanshupathak95

tests/phpunit/tests/admin/exportWp.php

@@ -302,7 +302,7 @@ public function test_export_wp_should_not_include_empty_comments_when_filtered()
 		// Add filter to make get_comment return null.
 		add_action(
 			'export_wp',
-			function () {
+			static function () {
 				add_filter( 'get_comment', '__return_null' );
 			}
 		);