How can I sort elements by numerical value of data attribute?

Use Array.sort:

var $wrapper = $('.testWrapper');

$wrapper.find('.test').sort(function(a, b) {
    return +a.dataset.percentage - +b.dataset.percentage;
})
.appendTo($wrapper);

Here's the fiddle: http://jsfiddle.net/UdvDD/

---

If you're using IE < 10, you can't use the dataset property. Use getAttribute instead:

var $wrapper = $('.testWrapper');

$wrapper.find('.test').sort(function(a, b) {
    return +a.getAttribute('data-percentage') - +b.getAttribute('data-percentage');
})
.appendTo($wrapper);

Here's the fiddle: http://jsfiddle.net/UdvDD/1/

$('.testWrapper').find('.test').sort(function (a, b) {
   return $(a).attr('data-percentage') - $(b).attr('data-percentage');
})
.appendTo('.testWrapper');