Run in your JavaScript console:

  document.cookie = 'optimizelyBuckets=' + escape('{"145285685":"145865258"}') + '; domain=.23andme.com'; document.location.reload(true);

The other values I found:

  69 -> escape('{"145285685":"145818631"}')
  99 -> escape('{"145285685":"145866265"}')
  149 -> escape('{"145285685":"145285686"}')
  299 -> escape('{"145285685":"145891045"}')

Edit: Actually, it looks like this no longer works. They may have noticed and stopped the A/B test.

You can empty the cart and run

  window.$.ajax({url:'/special_offer/49NOV2012/?json=true'})

Then click add to cart and once in your cart it will be $49.

I'm going to have to start looking at more websites' Optimizely codes!