Fix Occasional Failing Test

https://build.spring.io/browse/AMQP-MEIGHT-JOB1-164

Test verifies that prefetched messages are processed
when the container is stopped. However, the test does
not ensure that the prefetched messages are all queued
before stopping.

Wait until the queue size is > 3 and the first message
has been delivered to the listener (prefetch = 5).

Add diagnostic to the latch assertions.