Coverage update config (serde_until.rs)

[leodziki]

Description

Completed tests for nativelink/nativelink-config/src/serde_until.rs Bazel test and cargo test both passed. And Pre-commit passed in direnv and everything is perfect. Fixes #1401

Type of change

• [x] New feature (non-breaking change which adds functionality)
• [x] Breaking change (fix or feature that would cause existing functionality to not work as expected)

  How Has This Been Tested?

  Bazel test, cargo test and pre-commit

  Checklist

• [x] Tests added/amended
• [x] bazel test //... passes locally

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This change is 

[leodziki]

I would like to receive feedback on my PR. @aaronmondal , @MarcusSorealheis

[aaronmondal]

Could you remove the files that are unrelated to serde_utils.rs?