release-24.1: codeowners: extract GetTestOwner function into new public helper method

[rickystewart]

Backport 1/1 commits from #158548.

/cc @cockroachdb/release

Release justification: Non-production code changes

---

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 #158715 is compliant with backport policy

Confidence: high
Backward compatible: true
Explanation: The PR is for backporting changes and includes a release justification in the PR body stating 'Release justification: Non-production code changes'. According to the policy, changes affecting non-production code are exempt from the requirement of being a critical bug or implementing a feature flag. Furthermore, the involved file paths suggest that the modified code is related to internal command functionalities and helper methods on code ownership which are used mainly for development processes or tools.

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

[cockroach-teamcity]

This change is