Fix/aiokafka tests fixes (#4384)

* fix(aiokafka): close AIOKafkaProducer

* fix(aiokafka): use AsyncMock to fix unawaited coroutine warning

* docs(aiokafka): add CHANGELOG entry for test warning fixes

* fix formatting

* Apply suggestion from @xrmx

---------

Co-authored-by: Emídio Neto <9735060+emdneto@users.noreply.github.com>
Co-authored-by: Riccardo Magliocchetti <riccardo.magliocchetti@gmail.com>