Add acceptance tests for custom-html

app/assets/javascripts/discourse/helpers/custom-html.js.es6

@@ -1,7 +1,7 @@
 import { registerHelper } from 'discourse-common/lib/helpers';
 import PreloadStore from 'preload-store';
 
-const _customizations = {};
+let _customizations = {};
 
 export function getCustomHTML(key) {
   const c = _customizations[key];
@@ -15,6 +15,10 @@ export function getCustomHTML(key) {
   }
 }
 
+export function clearHTMLCache() {
+  _customizations = {};
+}
+
 // Set a fragment of HTML by key. It can then be looked up with `getCustomHTML(key)`.
 export function setCustomHTML(key, html) {
   _customizations[key] = html;