Fix generate_unions_case integration test

[viirya]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

Part of #1404. Currently generate_unions_case cannot pass for Rust cases archery --debug integration --run-flight --with-cpp=false --with-rust=true.

Describe the solution you'd like
archery --debug integration --run-flight --with-cpp=false --with-rust=true should pass without failure.

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A clear and concise description of any alternative solutions or features you've considered.

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