test: schedule WPT variants individually

[panva]

Schedule generated WPT paths through the Python test runner, including query variants that can be validated independently.

Currently, WPT wrappers run serially in the Python runner, but each wrapper can independently launch up to availableParallelism() - 1 specs. Lowering -j therefore does not limit that internal concurrency.

With this change, WPT tasks share the existing Python work queue and each task runs one variant at a time. On our overloaded CI workers, lowering -j now also limits WPT concurrency, giving us control over its CPU and memory pressure.

This also provides:

• Smaller scheduling units: available slots can pick up work across WPT suites instead of waiting for an entire wrapper to finish.
• Cheaper retries: rerun the failing path instead of repeating the whole suite.
• Useful Jenkins timings: generated paths have separate TAP identities, durations, and failures, making expensive or flaky tests easier to identify.

Labeled as flaky-test Issues and PRs involving tests that fail intermittently in CI. given this will either eliminate the flakes or allow us to identify individual specs that flake in Jenkins which so far we weren't able to figure out.

Note that this is not intended to lower the wall-time spent in WPTs, it's meant to have individual WPT specs behave as if they were our individual tests rather than one massive unit (like is the case with e.g. webcrypto)

Assisted-by: Codex

Closes: #51854

[nodejs-github-bot]

Review requested:

• @nodejs/web-standards

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.38%. Comparing base (c909c63) to head (88a00f1).
⚠️ Report is 10 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #65984      +/-   ##
==========================================
+ Coverage   90.17%   90.38%   +0.21%     
==========================================
  Files         771      771              
  Lines      265451   265487      +36     
  Branches    50459    50671     +212     
==========================================
+ Hits       239361   239951     +590     
+ Misses      17059    16487     -572     
- Partials     9031     9049      +18     

see 41 files with indirect coverage changes

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[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/77382/