Add test for Clear-Site-Data: "cache" in not-same-site origin.

[fred-wang]

This is a tentative test for https://bugs.webkit.org/show_bug.cgi?id=315416

[fred-wang]

I'm making this a draft because the test currently only passes in Firefox, so I might be missing some subtle server setup with WPT or browser caching behavior...

In WebKit, test Resource not in cache after clearing cache fails. This is expected because of https://bugs.webkit.org/show_bug.cgi?id=315416 but even with the patch there it's still failing for some reason?

In Chromium, test Resource in cache after first load fails, so it seems the image is never cached.

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Here are equivalent manual steps:

1. Go to https://wpt.live/ and open the devtools' console.
2. Execute await fetch("https://wpt.live/images/green-100x100.png?pipe=header(Cache-Control,max-age=3600)").then(_ => _.bytes()).then(_ => performance.getEntriesByType("resource").at(-1).transferSize) twice. It should return 0 the second time (i.e. resource is in cache).
3. Execute await fetch("/clear-site-data/support/echo-clear-site-data.py?cache").then(_ => _.text()) to clear the browser cache.
4. Execute command from step 2 again, it should return 40579 > 300 (i.e. resource is no longer in cache).

That time, we see expected behavior in Firefox and Chromium but not in WebKit. And we get expected behavior in WebKit with the patch for bug 315416 applied.