release-25.2: bazci: improve github issues when test.xml does not contain fully qualified unit test names

[williamchoe3]

Backport 1/1 commits from #159751 on behalf of @williamchoe3.

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Because of behavior described in #159708, certain failure modes can result in test.xml files that result in ambiguous github issues.

This change simply adds a message to the eventual github issue body so that test-eng or any team that's triaging can better see what's going on at a glance

Informs #159708

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Release justification: CI testing related change

[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 #159826 is compliant with backport policy

Confidence: high
Backward compatible: true
Explanation: This PR adds enhancements to the GitHub issue generation from test failure reports to include more descriptive messages. The changes are specific to a testing infrastructure tool used internally (indicated by 'pkg/cmd/' directory under non-production file patterns) and thus, fall under the exception for non-production and test-only changes. The files modified ('pkg/cmd/bazci/githubpost/githubpost.go', 'pkg/cmd/bazci/githubpost/githubpost_test.go', and 'pkg/cmd/bazci/githubpost/testdata/test-error-142.xml') support testing infrastructure and tools, exempting them from the standard backport policy requirements. Moreover, the PR contains a release justification indicating it's intended for CI testing related changes, confirming its alignment with the exceptions provided by the policy.

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

[github-actions]

Reminder: it has been 2 weeks please merge or close your backport!

[williamchoe3]

Rebasing / Rerunning CI since it's been a few weeks