Issue #2655336 by quietone: i18n Variable to config: site offline message [d6]

(cherry picked from commit 5ad41ae9)

core/modules/config_translation/migration_templates/d6_i18n_system_maintenance.yml

+id: d6_i18n_system_maintenance
+label: Maintenance page configuration
+migration_tags:
+  - Drupal 6
+source:
+  plugin: i18n_variable
+  variables:
+    - site_offline_message
+process:
+  langcode: language
+  message: site_offline_message
+destination:
+  plugin: config
+  config_name: system.maintenance

core/modules/config_translation/tests/src/Kernel/Migrate/d6/MigrateI18nSystemMaintenanceTest.php

+<?php
+
+namespace Drupal\Tests\config_translation\Kernel\Migrate\d6;
+
+use Drupal\Tests\migrate_drupal\Kernel\d6\MigrateDrupal6TestBase;
+
+/**
+ * Upgrade i18n maintenance variables to system.*.yml.
+ *
+ * @group migrate_drupal_6
+ */
+class MigrateI18nSystemMaintenanceTest extends MigrateDrupal6TestBase {
+
+  public static $modules = ['language', 'config_translation'];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp() {
+    parent::setUp();
+    $this->executeMigration('d6_i18n_system_maintenance');
+  }
+
+  /**
+   * Tests migration of system (maintenance) variables to system.maintenance.yml.
+   */
+  public function testSystemMaintenance() {
+    $config = \Drupal::service('language_manager')->getLanguageConfigOverride('fr', 'system.maintenance');
+    $this->assertIdentical('fr - Drupal is currently under maintenance. We should be back shortly. Thank you for your patience.', $config->get('message'));
+  }
+
+}