test_runner: do not reuse a worker ID held by a running file

[vserpokryl]

WorkerIdPool handed out worker IDs round-robin ((nextId++ % maxConcurrency) + 1)
and never released them, so an ID was only unique among the first N test files.
As soon as files finished out of order, a file that started later was given an ID
still held by a live process, which defeats the purpose of context.workerId:
allocating a database, port or directory per worker.

With --test-concurrency=2 and three files, where the first one outlives the
second:

start slow 1
start fast 2
end fast 2
start last 1   <-- same ID as the still-running 'slow'
end last 1
end slow 1

Track the IDs that are actually in use and hand out the lowest free one, then
release it once the child process is gone. The release happens in a finally
block so an aborted run or a failed spawn does not leak the ID out of the pool.

The pool no longer needs to be told the concurrency level. That also removes a
mismatch: the pool was sized from globalOptions.concurrency ?? concurrency,
which can differ from the concurrency the root test actually enforces. Since IDs
are handed out lowest-first, the highest ID in use is now bounded by how many
files really run at once.

Refs: #61394

[nodejs-github-bot]

Review requested:

• @nodejs/test_runner

[codecov]

Codecov Report

❌ Patch coverage is 97.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.99%. Comparing base (5ed55ba) to head (5416f6a).
⚠️ Report is 25 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/internal/test_runner/runner.js	97.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #65739      +/-   ##
==========================================
- Coverage   90.05%   89.99%   -0.06%     
==========================================
  Files         754      757       +3     
  Lines      256518   257374     +856     
  Branches    48534    48795     +261     
==========================================
+ Hits       231007   231627     +620     
- Misses      16609    16838     +229     
- Partials     8902     8909       +7     

Files with missing lines	Coverage Δ
lib/internal/test_runner/runner.js	95.19% <97.00%> (+0.24%)	⬆️

... and 52 files with indirect coverage changes

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