[test] Add testCase.next()

`testCase.next(fn)` is a wrapper for an all too familiar pattern of calling `test.wait()` and `test.resume()` after a callback. `test.next(fn)` creates a callback that automatically resumes the test, reducing the bureaucracy for asynchronous testing.

Example;

```js
// using test.resume()
Y.jsonp(uri, function (response) {
  test.resume(function () {
    Y.Assert.isObject(response);
  });
});
test.wait();

// using test.next()
Y.jsonp(uri, test.next(function (response) {
  Y.Assert.isObject(response);
}));
test.wait();
```