Fix MQTT test flake (backport #12687)

[mergify]

Prior to this commit, test

make -C deps/rabbitmq_mqtt ct-mqtt_shared t=[mqtt,cluster_size_1,v4]:non_clean_sess_reconnect_qos0_and_qos1

flaked in CI with error:

{mqtt_shared_SUITE,non_clean_sess_reconnect_qos0_and_qos1,972}
{badmatch,{publish_not_received,<<"msg-0">>}}

The problem was the following race condition:

• The MQTT v4 client sends an async DISCONNECT
• The global MQTT consumer metric got decremented. However, the classic queue still has the MQTT connection proc registered as consumer.
• The test case sends a message
• The classic queue checks out the message to the old connection instead of checking out the message to the new connection.

The solution in this commit is to check the consumer count of the classic queue before proceeding to send the message after disconnection.

---

This is an automatic backport of pull request #12687 done by [Mergify](https://mergify.com).