Add unit tests for asyncio producer

[fangnx]

What

Checklist

• Contains customer facing changes? Including API/behavior changes
• Did you add sufficient unit test and/or integration test coverage for this PR?
  • If not, please explain why it is not required

References

JIRA: https://confluentinc.atlassian.net/browse/DGS-21925

Test & Review

Open questions / Follow-ups

[confluent-cla-assistant]

🎉 All Contributor License Agreements have been signed. Ready to merge.
_{Please push an empty commit if you would like to re-run the checks to verify CLA status for all contributors.}

[sonarqube-confluent]

• 55.70% Coverage on New Code (is less than 80.00%)

Analysis Details

24 Issues

• 4 Bugs
• 0 Vulnerabilities
• 20 Code Smells

Coverage and Duplications

• 55.70% Coverage (65.90% Estimated after merge)
• No duplication information (5.30% Estimated after merge)

Project ID: confluent-kafka-python

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