test: keep finalization ref alive until exit

[trivikr]

Fixes an intermittent failure in parallel/test-process-finalization.

The different-registry-per-thread.mjs fixture registered an object with
process.finalization.register() but did not keep that object strongly
reachable until process exit. Since finalization stores weak references, GC
could collect the main-thread object before shutdown, causing the expected
shutdown on main thread output to be missing.

This keeps the registered reference reachable through the exit phase so the
test verifies per-thread finalization behavior without depending on GC timing.

Refs: https://github.com/nodejs/reliability/blob/main/reports/2026-05-15.md#jstest-failure

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Assisted-by: openai:gpt-5.5

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.05%. Comparing base (bc90667) to head (fc775f0).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #63394   +/-   ##
=======================================
  Coverage   90.05%   90.05%           
=======================================
  Files         714      714           
  Lines      225656   225704   +48     
  Branches    42696    42717   +21     
=======================================
+ Hits       203214   203267   +53     
- Misses      14220    14231   +11     
+ Partials     8222     8206   -16     

see 49 files with indirect coverage changes

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[trivikr]

Converted to draft, as the flaky test is likely fixed by #63209
It can be closed in future, if the test failures are not repeated.

The PRs for which the CI failed on https://github.com/nodejs/reliability/blob/main/reports/2026-05-15.md#jstest-failure did not contain the fix from #63209.