test: support inspecting WPTs in child processes

[panva]

This addresses the inspector debugging limitation described in #51854 and builds on the selectable thread/process WPT backend introduced in c757550.

WPT specs running in worker threads make attaching an inspector to an individual test difficult, this adds an opt-in WPT_INSPECT mode that:

• Forces the process backend
• Requires the selector to resolve to exactly one non-worker WPT variant
• Starts the child with --inspect-brk=0
• Forwards the debugger URL from child stderr
• Rejects dedicated-worker variants because the debugger attaches to the child-process harness, while their test code executes in a Worker created by that harness

Also document WPT execution backends (which I failed to do in c757550), source and generated test selectors, runner options, and the inspector workflow.

[nodejs-github-bot]

Review requested:

• @nodejs/web-standards

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.13%. Comparing base (453f47a) to head (0d5bf9e).
⚠️ Report is 21 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #65510      +/-   ##
==========================================
- Coverage   90.14%   90.13%   -0.01%     
==========================================
  Files         751      751              
  Lines      252709   253549     +840     
  Branches    47557    47764     +207     
==========================================
+ Hits       227800   228547     +747     
- Misses      16206    16256      +50     
- Partials     8703     8746      +43     

see 54 files with indirect coverage changes

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[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/76407/