tools: wait for test processes instead of polling

[mcollina]

RunProcess polled the child with poll() and an exponential backoff sleep capped at 100 ms, so every test longer than roughly 300 ms paid 30-75 ms of latency after it had already exited. Measured directly against a blocking wait on a setTimeout child:

child runtime	old Execute	blocking wait	added latency
300 ms	414 ms	380 ms	34 ms
1000 ms	1107 ms	1032 ms	75 ms
3000 ms	3115 ms	3088 ms	27 ms

This replaces the loop with a blocking wait(); a threading.Timer delivers the kill (SIGTERM, or SIGABRT with --abort-on-timeout) if the timeout is crossed, and wait() then returns the post-kill exit code exactly as before. The timer is cancelled as soon as the process exits.

Because workers now observe a child's exit with no delay, a child killed by the same ctrl-c that interrupts the runner could be recorded as a crashed test before the main thread set the shutdown flag, turning "Test aborted." into a "Failed tests:" list. The flag is now set from the SIGINT handler itself so it wins that race.

Parallel suite on an idle 8-core Linux box, -j8, no failures in either run:

	wall	sum of test durations	median test
before	2m56s	1338 s	137 ms
after	2m45s	1258 s	119 ms

Verified: timeout and --abort-on-timeout paths (exit_code -15 / -6, timed_out set), ctrl-c three times in a row aborts immediately with no leftover node processes, pseudo-tty/message/parallel samples pass, make lint-py clean.

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AI generated, reviewed by me.

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/78123/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.37%. Comparing base (8bf7793) to head (5c803f0).
⚠️ Report is 15 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #66432      +/-   ##
==========================================
- Coverage   90.39%   90.37%   -0.02%     
==========================================
  Files         792      792              
  Lines      275580   275697     +117     
  Branches    52840    52856      +16     
==========================================
+ Hits       249104   249169      +65     
- Misses      16897    16937      +40     
- Partials     9579     9591      +12     

see 43 files with indirect coverage changes

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[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/78128/