Test state partitioning in 3rd party iframes

[jonkoops]

Tests should be written to test access to 3rd party cookies and state partitioning. For more information see implementation in #14680.

[vmuzikar]

Just a couple of thoughts.

The partitioning should be fairly simple to trigger. All it takes is to set the hostname-admin to a different domain than frontend hostname. E.g.

./kc.sh start-dev --hostname=frontend-127-0-0-1.nip.io --hostname-admin=localhost

In this case, login flow will run on frontend-127-0-0-1.nip.io but the admin console (simulating a client app) will be on localhost.

We should also add some tests for the cookies fallback for when the storage API is unavailable. This might be a bit tricky and might require an older browser.

But in general, IMHO we should be testing with the latest (even beta/dev) browsers to know as soon as possible they restricted something even more, to avoid more issues like #14466 in the future.