[tests] make the function expression, an iife to run the test

R=neis@chromium.org

Review-Url: https://codereview.chromium.org/2690163004
Cr-Commit-Position: refs/heads/master@{#43180}

test/mjsunit/es6/promises.js

@@ -982,7 +982,7 @@ function assertAsyncDone(iteration) {
   var promise = new Promise(function(res) { resolve = res; });
   resolve({ then() { thenCalled = true; throw new Error(); } });
   assertLater(function() { return thenCalled; }, "resolve-with-thenable");
-});
+})();
 
 (function() {
   var calledWith;