test_runner: add --test-coverage flag

[mcollina]

Summary

• add a stable --test-coverage flag and keep --experimental-test-coverage as an alias
• update the test runner, docs, and config schema to use the stable flag as the canonical name
• filter the legacy alias from child run() propagation so coverage is not re-enabled in child tests

Testing

python3 tools/test.py \
  test/parallel/test-runner-flag-propagation.js \
  test/parallel/test-runner-coverage.js \
  test/parallel/test-runner-coverage-thresholds.js \
  test/parallel/test-runner-coverage-source-map.js \
  test/parallel/test-runner-coverage-default-exclusion.mjs \
  test/parallel/test-runner-cli.js \
  test/parallel/test-config-file.js \
  test/test-runner/test-output-typescript-coverage.mjs \
  test/test-runner/test-output-coverage-with-mock.mjs

Fix: #63506

[nodejs-github-bot]

Review requested:

• @nodejs/config
• @nodejs/test_runner

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.50%. Comparing base (df2ea49) to head (14a1fd3).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #63507      +/-   ##
==========================================
- Coverage   90.51%   90.50%   -0.02%     
==========================================
  Files         797      797              
  Lines      278252   278257       +5     
  Branches    53472    53463       -9     
==========================================
- Hits       251868   251832      -36     
- Misses      16761    16806      +45     
+ Partials     9623     9619       -4     

Files with missing lines	Coverage Δ
lib/internal/process/pre_execution.js	96.50% <100.00%> (-0.01%)	⬇️
lib/internal/test_runner/runner.js	95.14% <100.00%> (+<0.01%)	⬆️
lib/internal/test_runner/utils.js	66.30% <100.00%> (+0.13%)	⬆️
src/node_options.cc	81.92% <100.00%> (+0.09%)	⬆️

... and 32 files with indirect coverage changes

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[marco-ippolito]

Isnt it stable?

[MoLow]

LGTM