release-25.2: codeowners: extract GetTestOwner function into new public helper method

[rickystewart]

Backport 1/1 commits from #158548 on behalf of @rickystewart.

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We will use this in a new helper tool later.

Part of: DEVINF-1649
Release note: none
Epic: DEVINF-1582

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Release justification: Non-production code changes

[blathers-crl]

Thanks for opening a backport.

Before merging, please confirm that it falls into one of the following categories (select one):

• Non-production code changes. Includes test-only changes, build system changes, etc.
• Fixes for serious issues. Defined in the policy as correctness, stability, or security issues, data corruption/loss, significant performance regressions, breaking working and widely used functionality, or an inability to detect and debug production issues.
• Other approved changes. These changes must be gated behind a disabled-by-default feature flag unless there is a strong justification not to.

Add a brief release justification to the PR description explaining your selection.

Also, confirm that the change does not break backward compatibility and complies with all aspects of the backport policy.

All backports must be reviewed by the TL and EM for the owning area.

[cockroach-teamcity]

This change is 

[blathers-crl]

❌ PR #158710 does not comply with backport policy

Confidence: high
Explanation: The pull request introduces modifications to production code, specifically within helper tools and internal functions, as evidenced by the changes in 'pkg/cmd/bazci/githubpost/githubpost.go' and 'pkg/internal/codeowners/codeowners.go'. These modifications do not fulfill the critical bug criteria specified in the policy. The changes primarily include the extraction and refactoring of methods related to testing ownership determination, and the integration of a new helper function into existing tools. There is no evidence of the changes addressing stability or security issues, data loss, significant performance regressions, or corrections of incorrect database results. The code changes do not implement any feature flags or cluster settings that would enable gating of these changes, nor meet the criteria to be considered as such.
Recommendation: Reconsider the backport or introduce a feature flag to gate these changes

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