test: regression test based on #7006

A test based on the description in #7006

1. Start cluster master, worker
2. Connect to cluster with separate client
3. Worker dies badly
4a. Client gets disconnected (reliably on v0.10)
4b. Client does not get disconnected (frequent on v0.11)

Seems to always pass on v0.10, but intermittently fail on v0.11. Having random worker deaths result in hung clients makes cluster a lot less appealing.