Fix config menu route

- Fixes #56

message_subscribe.links.menu.yml

@@ -2,4 +2,4 @@ message_subscribe.admin_settings:
   route_name: message_subscribe.admin_settings
   title: 'Message subscribe settings'
   description: 'Administer message subscribe'
-  parent: system.admin_config_system
+  parent: message.main_settings

message_subscribe.routing.yml

@@ -1,5 +1,5 @@
 message_subscribe.admin_settings:
-  path: /admin/config/system/message-subscribe
+  path: /admin/config/message/message-subscribe
   defaults:
     _title: 'Message subscribe settings'
     _form: \Drupal\message_subscribe\Form\MessageSubscribeAdminSettings

tests/src/Functional/MenuTest.php

@@ -0,0 +1,39 @@
+<?php
+
+namespace Drupal\Tests\message_subscribe\Functional;
+
+use Drupal\Core\Url;
+use Drupal\Tests\BrowserTestBase;
+
+/**
+ * Tests menus for the message subscribe module.
+ *
+ * @group message_subscribe
+ */
+class MenuTest extends BrowserTestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public static $modules = ['message_subscribe'];
+
+  /**
+   * Test that the menu links are working properly.
+   */
+  public function testMenuLinks() {
+    $admin = $this->drupalCreateUser([], NULL, TRUE);
+    $this->drupalLogin($admin);
+
+    // Link should appear on main config page.
+    $this->drupalGet(Url::fromRoute('system.admin_config'));
+    $this->assertSession()->linkExists(t('Message subscribe settings'));
+
+    // Link should be on the message-specific overview page.
+    $this->drupalGet(Url::fromRoute('message.main_settings'));
+    $this->assertSession()->linkExists(t('Message subscribe settings'));
+
+    $this->clickLink(t('Message subscribe settings'));
+    $this->assertSession()->statusCodeEquals(200);
+  }
+
+}