[ticket/17176] Fix some PHP 8.x deprecations and clean up test code

PHPBB3-17176

tests/dbal/migrator_test.php

@@ -42,6 +42,9 @@ class phpbb_dbal_migrator_test extends phpbb_database_test_case
 	/** @var \phpbb\config\config */
 	protected $config;
 
+	/** @var \phpbb\extension\manager */
+	protected $extension_manager;
+
 	public function getDataSet()
 	{
 		return $this->createXMLDataSet(__DIR__.'/fixtures/migrator.xml');