Test case for https://bugs.webkit.org/show_bug.cgi?id=30457. Previously, if an image
load was trigger from an unload handler, we would kill it almost immediately due to the
navigation stopping all loaders. These loads now happen entirely in the background and detached
from the DOM, so they're invisible to the normal resource load callback infrastructure. We generate a
timestamp, then in the unload handler, we load an 'image' (actually a php script) that takes the
timestamp as a parameter and saves it to a file. The destination page is a php script that checks for
the existence of that file and passes the test if the file contains the expected timestamp.