tests: fix shm test deadlock

Stopping the consumer first would occasionally allow the producer to fill the shm segment causing it to block in send() and unable to be stopped.

This is the easy fix and works around the issue. Probably the correct long term fix is to shutdown/signal the pipe on stop or something but I started to get lost trying to figure out all the pieces. If somebody can easily fix that or point me in the correct location I'll take another look.