Fix failing bookmark tests

src/headless/plugins/bookmarks/collection.js

 import "@converse/headless/plugins/muc/index.js";
+import Bookmark from './model.js';
 import log from "@converse/headless/log.js";
 import { __ } from 'i18n';
 import { _converse, api, converse } from "@converse/headless/core";
@@ -9,6 +10,7 @@ const u = converse.env.utils;
 
 const Bookmarks = {
 
+    model: Bookmark,
     comparator: (item) => item.get('name').toLowerCase(),
 
     initialize () {
@@ -25,10 +27,6 @@ const Bookmarks = {
         this.browserStorage = _converse.createStore(cache_key);
     },
 
-    model (attrs, options) {
-        return new _converse.Bookmark(attrs, options);
-    },
-
     async openBookmarkedRoom (bookmark) {
         if ( api.settings.get('muc_respect_autojoin') && bookmark.get('autojoin')) {
             const groupchat = await api.rooms.create(

src/headless/plugins/bookmarks/index.js

@@ -102,7 +102,7 @@ converse.plugins.add('converse-bookmarks', {
 
         api.promises.add('bookmarksInitialized');
 
-        _converse.Bookmark = Model.extend(Bookmark);
+        _converse.Bookmark = Bookmark;
         _converse.Bookmarks = Collection.extend(Bookmarks);
 
         _converse.BookmarksList = Model.extend({

src/headless/plugins/bookmarks/model.js

 import { converse } from '@converse/headless/core';
+import { Model } from '@converse/skeletor/src/model.js';
 
 const { Strophe } = converse.env;
 
-const Bookmark = {
+const Bookmark = Model.extend({
     idAttribute: 'jid',
     getDisplayName () {
         return Strophe.xmlunescape(this.get('name'));
     }
-};
+});
 
 export default Bookmark;