One callback per live server that tells its connected pages to drop the rest of the QUnit queue — what qq, session.abort() and an aborted signal all go through.

A set rather than a slot because a concurrent run may have one server per group, and aborting half a run is not aborting it. Shared by reference across groups, so a group registering its own server marks it for everyone. Entries for closed servers are harmless: publishing to a server with no clients reaches nobody.