Issue #2905720 by vaplas: Media specifies a add-page link relation, but...

Issue #2905720 by vaplas: Media specifies a add-page link relation, but core.link_relation_types does not have this

core/core.link_relation_types.yml

@@ -3,6 +3,9 @@
 add-form:
   uri: https://drupal.org/link-relations/add-form
   description: A form where a resource of this type can be created.
+add-page:
+  uri: https://drupal.org/link-relations/add-page
+  description: A page where a resource of this type and related types can be created.
 delete-form:
   uri: https://drupal.org/link-relations/delete-form
   description: A form where a resource of this type can be deleted.

core/modules/media/tests/src/Kernel/MediaLinkRelationsTest.php

+<?php
+
+namespace Drupal\Tests\media\Kernel;
+
+use Drupal\media\Entity\Media;
+
+/**
+ * Tests link relationships for Media entity.
+ *
+ * @group media
+ */
+class MediaLinkRelationsTest extends MediaKernelTestBase {
+
+  /**
+   * Tests that all link relationships for Media exist.
+   */
+  public function testExistLinkRelationships() {
+    /** @var \Drupal\Core\Http\LinkRelationTypeManager $link_relation_type_manager */
+    $link_relation_type_manager = $this->container->get('plugin.manager.link_relation_type');
+    $media = Media::create(['bundle' => $this->testMediaType->id()]);
+    $media->save();
+    foreach ($media->uriRelationships() as $relation_name) {
+      $this->assertTrue($link_relation_type_manager->hasDefinition($relation_name), "Link relationship '{$relation_name}' for Media entity");
+    }
+  }
+
+}