[feature/migrations] Function effectively_installed() in migrations

Allows you to check if the migration is effectively installed
(entirely optionall)

This function is intended to help moving to migrations from a
previous database updater, where some migrations may have been
installed already even though they are not yet listed in the
migrations table.

PHPBB3-9737

tests/dbal/migration/installed.php

@@ -0,0 +1,30 @@
+<?php
+/**
+*
+* @package testing
+* @copyright (c) 2011 phpBB Group
+* @license http://opensource.org/licenses/gpl-license.php GNU Public License
+*
+*/
+
+class phpbb_dbal_migration_installed extends phpbb_db_migration
+{
+	function effectively_installed()
+	{
+		return true;
+	}
+
+	function update_data()
+	{
+		return array(
+			array('custom', array(array(&$this, 'test'))),
+		);
+	}
+
+	function test()
+	{
+		global $migrator_test_installed_failed;
+
+		$migrator_test_installed_failed = true;
+	}
+}