tests: add waitForPromise method and test for it

tests/frontend/helper.js

@@ -197,6 +197,17 @@ var helper = {};
     return deferred;
   };
 
+  /**
+   * Same as `waitFor` but using Promises
+   *
+   */
+  helper.waitForPromise = async function(...args) {
+    // Note: waitFor() has a strange API: On timeout it rejects, but it also throws an uncatchable
+    // exception unless .fail() has been called. That uncatchable exception is disabled here by
+    // passing a no-op function to .fail().
+    return await this.waitFor(...args).fail(() => {});
+  };
+
   helper.selectLines = function($startLine, $endLine, startOffset, endOffset){
     // if no offset is provided, use beginning of start line and end of end line
     startOffset = startOffset || 0;