Preload and image tests

I added a couple of interesting aspects to this test:
* Added an image to the page, so we can also see if inapplicable CSS is delaying the download of images
* Added an identical test, only with a `<script`> tag before the CSS, so that the preload parser will kick in.

Haven't throughly tested it yet, but at least in FF I see that preloading inapplicable CSS is delaying images.