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

• New feature (non-breaking change which adds functionality)
• 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

• Tests added/amended
• 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?