Fix flaky AmqpchannelProcessor test

[lmolkova]

Description

Please add an informative description that covers that changes made by the pull request and link all relevant issues.

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All SDK Contribution checklist:

• The pull request does not introduce [breaking changes]
• CHANGELOG is updated for new features, bug fixes or other significant changes.
• I have read the contribution guidelines.

General Guidelines and Best Practices

• Title of the pull request is clear and informative.
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Testing Guidelines

• Pull request includes test coverage for the included changes.

[azure-sdk]

API change check

API changes are not detected in this pull request.

[lmolkova]

Run with the test repeated 1000 times with no issues - https://dev.azure.com/azure-sdk/public/_build/results?buildId=3479167&view=results