This changes the test a bit. It doesn't test BroadcastChannel's postMessage anymore, but receiving the message.
I can't see how postMessage could be tested, since given the new setup, no one should have access to it anymore.
This is functionally now very similar to another test I wrote recently, but I think it is fine to keep this too.
Differential Revision: https://phabricator.services.mozilla.com/D109539
