These test were very sensible to the speed at which ZeroMQ distributes
subscriptions in the cluster and showed to be unreliably when testing with
zeek/btest#113.
The main fix here is to have individual WebSocket clients subscribe to unique
topics, e.g /test/client-0 and /test/client-1, instead of just a shared topic.
This ensures the WebSocket handshake completes only when they observed their
own subscriptions and not prematurely when observing the shared topic.
This seems mainly relevant for tests: In the real world one shouldn't
rely on subscription visibility - you miss messages if you're too late
to the party.
