Aggregate JUnit results at parent test level (#6238)

[Akshat005Chaudhary]

Aggregate JUnit results at parent test level

Closes #6238

Background

Currently, child TAP files are already aggregated and surfaced at the parent build level. However, JUnit XML results (produced by openjdk/JCK tests) are only visible at the child job level — engineers have to click into each parallel testList_N job individually to see test reports.

This PR brings JUnit result aggregation to the parent build level, matching the existing TAP behaviour.

Changes

buildenv/jenkins/JenkinsfileBase

Two additions inside archiveChildJobTap():

1. Per-child JUnit artifact copy — alongside the existing **/*.tap copy, also fetches JUnit XML files from each parallel child job into the parent workspace:

copyArtifacts(projectName: "${name}", selector: specific("${buildId}"),
    filter: "**/work/**/*.jtr.xml, **/result/**/*.jtr.xml, **/junitreports/**/*.xml, **/external_test_reports/**/*.xml",
    target: "${name}/${buildId}", optional: true)

optional: true ensures child jobs that produce no JUnit output (e.g. non-openjdk tests) are silently skipped without failing the step.

2. Parent-level junit publish step — after all child artifacts are gathered, publishes the aggregated report at the parent build:

junit skipMarkingBuildUnstable: true, allowEmptyResults: true, keepLongStdio: true,
      testResults: '**/work/**/*.jtr.xml, **/result/**/*.jtr.xml, **/junitreports/**/*.xml, **/external_test_reports/**/*.xml'

Safety / Status Interference

skipMarkingBuildUnstable: true is used intentionally to ensure the junit step never overrides the pipeline's own build status logic (checkTestResults()). This directly addresses the concern raised in #6244, where the TAP plugin was removed precisely because it forcibly set job status to UNSTABLE — this implementation avoids repeating that mistake.

allowEmptyResults: true ensures the parent build doesn't fail when no JUnit XML is present (e.g. platforms that only produce TAP).

Testing

• Verified JUnit report appears at parent build level after a parallel test run
• Verified build status is not affected by the junit step (remains controlled by checkTestResults())
• Verified child jobs with no JUnit output are handled gracefully