[FIX] website_slides: Display button when not in fullscreen

When answering a quiz, the button "Check your answers"
is not displayed when not in fullscreen.

JS error: `def.resolve is not a function`
because an already instantiated Promise does not have a
resolve function.
A promise cannot be resolved from outside its callback scope.

closes odoo/odoo#34762

Signed-off-by: Yannick Tivisse (yti) <yti@odoo.com>

addons/website_slides/static/src/js/slides_course_quiz.js

@@ -57,13 +57,11 @@ odoo.define('website_slides.quiz', function (require) {
          * @override
          */
         willStart: function () {
-            var def = new Promise(function () {});
-            if (this.quiz) {
-                def.resolve();
-            } else {
-                def = this._fetchQuiz();
+            var defs = [this._super.apply(this, arguments)];
+            if (!this.quiz) {
+                defs.push(this._fetchQuiz());
             }
-            return Promise.all([this._super.apply(this, arguments), def]);
+            return Promise.all(defs);
         },
 
         /**