test_runner: avoid waiting on inherited stdio with force exit

[dayun6530]

Fixes: #66336

When --test-force-exit is used, the parent test runner waits for both
the test file process to exit and its stdout stream to finish.

If the test file leaks a child process with inherited stdio, that child
keeps the stdout pipe open even after the test file process exits. This
causes the test runner to remain alive until the leaked child exits.

Use the test:summary event as an additional completion signal when
--test-force-exit is enabled. Once the summary has been received, the
test report has completed and the parent no longer needs to wait for EOF
from stdout.

The existing stdout completion path is kept as a fallback.

A regression test is included for a leaked child process with inherited
stdio.

[nodejs-github-bot]

Review requested:

• @nodejs/test_runner

[addaleax]

If the test file leaks a child process with inherited stdio, that child
keeps the stdout pipe open even after the test file process exits. This
causes the test runner to remain alive until the leaked child exits.

That can happen regardless of the forced exit flag, right?

[codecov]

Codecov Report

❌ Patch coverage is 86.36364% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.35%. Comparing base (296584b) to head (449b988).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/internal/test_runner/runner.js	86.36%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #66390      +/-   ##
==========================================
- Coverage   90.36%   90.35%   -0.02%     
==========================================
  Files         792      792              
  Lines      275490   275519      +29     
  Branches    52796    52800       +4     
==========================================
- Hits       248954   248935      -19     
- Misses      16936    16988      +52     
+ Partials     9600     9596       -4     

Files with missing lines	Coverage Δ
lib/internal/test_runner/runner.js	95.02% <86.36%> (-0.17%)	⬇️

... and 28 files with indirect coverage changes

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