Add test for flushing a queue with outstanding in-flight messages

[relud]

1. create a queue with a disk location
2. write a message to the queue
3. call get to make the message in-flight
   • don't ack or nack the message
4. attempt to flush the queue from scratch
   • don't reuse or del the existing queue object
5. assert that the message was delivered

This should catch regressions of https://github.com/peter-wangxu/persist-queue/pull/154