test_runner: drain event loop before force-exit process.exit()

[NeverBetterEnough]

When --test-force-exit is used with --test-concurrency > 1, the parent process can silently lose test verdicts from child processes. This happens because process.exit() fires immediately after reporter destinations close, before the event loop has a chance to drain pending write operations on child-process report streams.

The previous fix (#55099) ensured reporter destination files are flushed before exit, but did not extend the flush guarantee to the child-to-parent report stream layer. With process isolation and concurrency, V8-serialized test-event frames from children can be truncated or dropped before the parent's FileTest parser consumes them.

This change adds a setImmediate tick before process.exit() to allow the event loop one full iteration to drain any remaining write operations on reporter destinations (including child-process stdout pipes).

Changes

• lib/internal/test_runner/test.js: Added await new Promise((resolve) => setImmediate(resolve)) before process.exit() in the force-exit handler
• test/parallel/test-runner-force-exit-concurrency.mjs: New regression test (8 files × 10 tests, run 5 times to catch the race)

Reproduction

The issue requires --test-force-exit + --test-concurrency > 1 (process isolation). With many fast test files, the parent silently loses a fraction of test verdicts — both built-in and custom reporters receive fewer events than tests that actually ran, yet the run still exits 0.

Impact

Silent partial results defeat the purpose of a test run. On a production suite (Node v24.18.0, 1351 tests), the parent reported as few as 1168. CI stays green while a meaningful fraction of the suite is unaccounted for.

Fixes: #64833
Refs: #54327, #55099

[nodejs-github-bot]

Review requested:

• @nodejs/test_runner

[avivkeller]

@NeverBetterEnough Please sign your commit by adding a Signed-off-by: trailer

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.15%. Comparing base (c8e2a82) to head (db862f5).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #64857      +/-   ##
==========================================
- Coverage   90.16%   90.15%   -0.01%     
==========================================
  Files         746      746              
  Lines      242760   242766       +6     
  Branches    45765    45757       -8     
==========================================
- Hits       218875   218863      -12     
- Misses      15375    15402      +27     
+ Partials     8510     8501       -9     

Files with missing lines	Coverage Δ
lib/internal/test_runner/test.js	97.93% <100.00%> (+<0.01%)	⬆️

... and 34 files with indirect coverage changes

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