add working test

tests/jsPsych.pluginAPI/preloads.test.js

@@ -0,0 +1,24 @@
+const root = '../../';
+const utils = require('../testing-utils.js');
+
+beforeEach(function(){
+  require(root + 'jspsych.js');
+  require(root + 'plugins/jspsych-html-keyboard-response.js');
+});
+
+describe('getAutoPreloadList', function(){
+  test('works with images', function(){
+    require(root + 'plugins/jspsych-image-keyboard-response.js');
+
+    var t = {
+      type: 'image-keyboard-response',
+      stimulus: 'img/foo.png'
+    }
+
+    var timeline = [t];
+
+    var images = jsPsych.pluginAPI.getAutoPreloadList(timeline).images;
+
+    expect(images[0]).toBe('img/foo.png');
+  })
+})