test_runner: add classname hierarchy for JUnit reporter

[mag123c]

Fixes: #59417

Implements classname hierarchy for the JUnit XML reporter.
Previously, all tests had classname="test".
Now the classname reflects the suite hierarchy (e.g., classname="Math.Addition" for nested suites).

[nodejs-github-bot]

Review requested:

• @nodejs/test_runner

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.57%. Comparing base (e105e82) to head (f738875).
⚠️ Report is 16 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #60220      +/-   ##
==========================================
+ Coverage   88.53%   88.57%   +0.04%     
==========================================
  Files         704      704              
  Lines      208087   208140      +53     
  Branches    40006    40013       +7     
==========================================
+ Hits       184223   184358     +135     
+ Misses      15869    15808      -61     
+ Partials     7995     7974      -21     

Files with missing lines	Coverage Δ
lib/internal/test_runner/test.js	97.39% <100.00%> (+0.02%)	⬆️
lib/internal/test_runner/tests_stream.js	90.11% <100.00%> (+0.11%)	⬆️

... and 41 files with indirect coverage changes

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[CuriousStork]

Hello @MoLow @pmarchini

Could you take a look? Thank you.

[mcollina]

lgtm

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/70930/