Support asynchronous test setup

[0x24a537r9]

I want to judge the relative performance of some image-blurring algorithms in JS, but there's not really a great way in jsperf, since I need the image to already be downloaded before the test starts (since the image-loading is not a part of my scenario). I see that async test cases are already supported. It would be nice to also have async setup/teardown.

In the meantime, I guess I just have to make the first case a dummy case that wastes enough time so as to allow the image to finish loading.