release-24.3: codeowners: extract GetTestOwner function into new public helper method

[rickystewart]

Backport 1/1 commits from #158548.

/cc @cockroachdb/release

---

We will use this in a new helper tool later.

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

[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.

[blathers-crl]

❌ PR #158714 does not comply with backport policy

Confidence: high
Explanation: The pull request involves changes to files that do not solely fall within non-production patterns specified by the policy. Specifically, the changes in pkg/internal/codeowners/codeowners.go and pkg/cmd/bazci/githubpost/githubpost.go suggest modifications to potentially production or core utility functionalities, which are not solely related to testing, development environment, or build configuration. There is no evidence of a 'Release justification' line in the PR body, indicating these changes need to be critically justified or gated behind a feature flag. The body of the PR does not claim that these changes fix a critical bug, nor do they appear to be related to addressing stability, security issues, or data corruption/loss, or address any performance or incorrect database results. Moreover, no new cluster or session settings are introduced which can act as feature flags, nor is there any apparent gating mechanism for these changes being presented.
Recommendation: Request to reconsider the backport or provide a 'Release justification' explaining why these changes should be backported without being gated behind a feature flag.

_{🦉 Hoot! I am a Blathers, a bot for CockroachDB. My owner is dev-inf.}

[cockroach-teamcity]

This change is