add test for sasl ssl

[LizardWizzard]

Changes

Hello! Thank you for great library! Just wanted to test if sasl_ssl auth method is working correctly while setting it up for my project. Realised that this scenario is missing in library's functional tests. So I've added it.

By the way, when tried to run tests on master there was a bunch of warnings like RuntimeWarning: coroutine 'TestKafkaClientIntegration.test_failed_bootstrap' was never awaited so many tests didnt actually run. When tried on top of the latest release tests run smoothly

Checklist

• [x] I think the code is well written
• [x] Unit tests for the changes exist
• [x] Documentation reflects the changes
• [ ] Add a new news fragment into the CHANGES folder
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    • .misc: A ticket has been closed, but it is not of interest to users.

[CLAassistant]

All committers have signed the CLA.

[tvoinarovskyi]

Thanks for the contribution! I remember having issues with the SSL+SASL scenario, because we create certificates for SSL in a very simple manner, not sure what was the specific problem.