AMQP-342: Fix Listener Integration Tests

JIRA: https://jira.springsource.org/browse/AMQP-342

Previously, the `MessageListenerContainerLifecycleIntegrationTests` relied
on Consumers' logic, where they can emit messages even after stop() of container.

Since AMQP-338 fix Consumers consumers are marked to stop on container stop().
So, now non-transactional consumers work as transactional and there is no received
new messages from message broker after stop(). Prefetched messages are processed
after the stop, however.

AMQP-342: Polishing

Just only test that messages aren't received after stop()

AMQP-342 Polishing - Add Tests for Non-Tx Prefetch

Test cases did not cover prefetch without transactions.

Remove obsolete comment.

Add assertion for txSize > 0 (otherwise container never
called listener).