[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();
```
[test] Add unit test for test.next()
[test] Add HISTORY entry for test.next()
Merge branch 'testcase-next' into dev-3.x