Adding unit tests for mutable fips create update test

[pineapplethevoyager]

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• Have you run azdev style <YOUR_EXT> locally? (pip install azdev required)
• Have you run python scripts/ci/test_index.py -q locally? (pip install wheel==0.30.0 required)
• My extension version conforms to the Extension version schema

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• My extension description/summary conforms to the Extension Summary Guidelines.

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Once your pull request is merged into main branch, a new pull request will be created to update src/index.json automatically.
You only need to update the version information in file setup.py and historical information in file HISTORY.rst in your PR but do not modify src/index.json.

[azure-client-tools-bot-prd]

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[yonzhan]

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For more info about extension versioning, please refer to Extension version schema