Remaining COOP and COEP testing work

[annevk]

Already done:

• Update SharedArrayBuffer tests to account for COOP/COEP #17690 - aligning all existing SharedArrayBuffer tests (see also Raise the bar for SharedArrayBuffer via postMessage() whatwg/html#4734)
• COOP and COEP tests #17606 - updating existing COOP and COEP tests and adding a bunch

To do:

• ImageBitmap (in particular an ImageBitmap created from an opaque response outside an COOP+COEP environment needs to result in a messageerror when messaged into a COOP+COEP environment). (I realize now that this is only really possible with BroadcastChannel or storage so HTML should not modify the transferable steps for this object.) HTML: COEP and ImageBitmap #18543
• COOP and redirects:
  • Final URL is not HTTP. Not sure if this can be tested as "blob" is forbidden, "data" in a top-level does not work in a lot of browsers. Though maybe final URL being a network error of sorts is sufficient.
  • Final response synthesized by a ServiceWorker (with COOP/COEP or with CO*P in intermediate, but not in final response).
  • COOP document detecting a late <meta charset> requiring the parser to change the encoding while parsing, and re-navigate.
• Sandboxing and COOP and COEP:
  • Framing URIs in non-"allow-same-origin" frames. COOP: test iframe sandbox without allow-same-origin #21109
  • COOP in sandboxed popups COOP: test COOP popup from a CSP-sandboxed popup #21111
  • COEP in sandboxed popups
  • COOP+COEP in sandboxed popups
• Opening blob popups for COOP/COEP (should inherit?)
• Non-initial about:blank (popup with cross-origin iframe that navigates the parent)

To do with lower priority:

• COEP + workers. At least Firefox will likely ship this later.
  • With service workers: Add cross-origin-embedder-policy tests with service workers #18559
  • With shared workers: [html] Test COEP derivation in shared workers #20899
  • (There should be some coverage for dedicated workers already.)
• Perhaps merge script-factory.js (and callers) and sandbox.https.html as they have a lot of similarities and that could lead to better coverage.

#17836 tracks ftp: support in web-platform-tests. If that somehow arrives in time we should add tests for that as well.

cc @mystor @JuniorHsu @yutakahirano @csreis @mikewest @mozfreddyb