These tests exercise differnt ways to load an image, generated via /referrer-policy/generic/subresource/image.py?id=<UUID> and later verify the headers used to request that image.

Since there is no way to wait for a resource referenced from CSS to be loaded, all tests use step_timeout() to delay the verification step until after the image (hopefully) was loaded.

Since there is also no way to retrieve headers (or other information) from images loaded via CSS, we store the headers with the given UUID as key on the server, and retrieve them later via an XHR to /referrer-policy/generic/subresource/image.py?id=<UUID>&report-headers.