rust: Fix package test

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Merge Checklist

All boxes should be checked before merging the PR (just tick any boxes which don't apply to this PR)

• The toolchain has been rebuilt successfully (or no changes were made to it)
• The toolchain/worker package manifests are up-to-date
• Any updated packages successfully build (or no packages were changed)
• All package sources are available
• cgmanifest files are up-to-date and sorted (./cgmanifest.json, ./toolkit/tools/cgmanifest.json, ./toolkit/scripts/toolchain/cgmanifest.json)
• LICENSE-MAP files are up-to-date (./SPECS/LICENSES-AND-NOTICES/LICENSES-MAP.md, ./SPECS/LICENSES-AND-NOTICES/LICENSE-EXCEPTIONS.PHOTON)
• All source files have up-to-date hashes in the *.signatures.json files
• sudo make go-tidy-all and sudo make go-test-coverage pass
• Documentation has been updated to match any changes to the build system
• [] Ready to merge

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Summary

Rust package tests were failing due to a missing BR and a bad test- this PR fixes both.

Change Log

• Add conditional dependency for rust on python-xml when running package tests
• Removed a low signal-to-noise ratio test from Rust. This test was removed in version 1.40.0- more context can be found in run-make-fulldeps/linker-output-non-utf8 failure rust-lang/rust#63520

Does this affect the toolchain?

NO

Test Methodology

• Local build with RUN_CHECK=y

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Opting to ignore remainder of linter suggestions.