test: ignore late worker exceptions in WPT runner

[marcopiraccini]

Problem

WPT semantics: once done() is called the test result is final. Browsers send any later exception to window.onerror and do not fail the test. Node's WPT worker forwarded post-completion uncaughtExceptions to the parent as Worker 'error' events, which the runner reports as failures.

Affected flaky tests:

• wpt/test-timers → html/webappapis/timers/type-long-settimeout.any.js
• wpt/test-timers → html/webappapis/timers/type-long-setinterval.any.js

Fix

Install a no-op uncaughtException handler from inside add_completion_callback, before posting completion. Late exceptions in the worker are then swallowed, matching browser semantics. Scope is one fixture per worker (one Worker per spec), so it cannot leak into other tests.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.67%. Comparing base (b2f6aa3) to head (fa4c14d).
⚠️ Report is 7 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #63106      +/-   ##
==========================================
+ Coverage   89.65%   89.67%   +0.02%     
==========================================
  Files         712      712              
  Lines      220512   220822     +310     
  Branches    42289    42385      +96     
==========================================
+ Hits       197690   198019     +329     
+ Misses      14663    14613      -50     
- Partials     8159     8190      +31     

see 52 files with indirect coverage changes

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