This should mean that logs from worker processes are flushed before shutdown.
When a test completes, Complement stops the docker container, which means that
synapse will receive a SIGTERM. Currently, the `complement_fork_starter` exits
immediately (without notifying the worker processes), which means that the
workers never get a chance to flush their logs before the whole container is
vaped. We can fix this by propagating the SIGTERM to the children.
