Feature request: Automatic test.done() call in asynchronous tests

Nodeunit has great feature in expect, but ending asynchronous tests is often painful, mainly in tests with more than 1 callback. I think, it has easy solution and implementation isn't hard.

Instread of passing raw callback, pass returned callback from `test.callback([expect 1,] callback);`.

Invoking `test.callback` will store given callback and mark him as done after expected number of calls (internal mechanism will watch status of each callback). Each call will invoke original callback. If all test.callback are marked as done, test.done is called automatically.

It should handle all cases, even callback in callback.

It doesn't break existing code, it is optional.

Implementation could look like this:

``` javascript
test.callback = function(expected, callback) {
if (typeof expected === 'function') {
callback = expected;
expected = 1;
}

this.callbacks = this.callbacks || 0;
this.callbacks++;

var test = this;
var done = false;

return function() {
expected--;
if (done===false && expected<=0) {
done = true;
test.callbacks--;
}

var ret = callback.apply(this, arguments);

if (test.callbacks<=0) {
test.done();
}

return ret;
};
}
```

Empieza localizando el manejo de pruebas asíncronas y los puntos de entrada existentes test.callback y test.done. Sigue cómo se registran y completan actualmente los callbacks y, después, comprueba los casos propuestos que implican varios callbacks y callbacks anidados. Se considera terminado cuando el wrapper de callback opcional conserva el comportamiento original del callback e invoca test.done solo después de que se completen todos los callbacks esperados.

Escrito por el modelo de indexación a partir del texto del issue.