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 89.80%. Comparing base (c1402c6) to head (175d312).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #60220      +/-   ##
==========================================
+ Coverage   89.77%   89.80%   +0.02%     
==========================================
  Files         672      672              
  Lines      203930   203944      +14     
  Branches    39207    39213       +6     
==========================================
+ Hits       183082   183151      +69     
+ Misses      13175    13116      -59     
- Partials     7673     7677       +4     

Files with missing lines	Coverage Δ
lib/internal/test_runner/test.js	97.33% <100.00%> (+0.02%)	⬆️
lib/internal/test_runner/tests_stream.js	89.77% <100.00%> (+0.11%)	⬆️

... and 37 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/

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/71009/