test: keep finalization before-exit ref alive

[pimterry]

This is a fixture within the test-process-finalization test, which flaked yesterday, and quite a bit recently: https://github.com/nodejs/reliability/issues?q=%22test-process-finalization%22. All the examples for the recent failures are from this fixture.

Exactly the same problem & fix as for the sibling test/fixtures/process/close.mjs fixture: #64085. I have checked and AFAICT this is the last remaining example of the set, so hopefully this'll make test-process-finalization stable once and for all 🤞

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/74841/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.24%. Comparing base (824bc66) to head (e5bbbdd).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #64521   +/-   ##
=======================================
  Coverage   90.24%   90.24%           
=======================================
  Files         739      739           
  Lines      241653   241639   -14     
  Branches    45539    45544    +5     
=======================================
+ Hits       218073   218076    +3     
+ Misses      15111    15109    -2     
+ Partials     8469     8454   -15     

see 33 files with indirect coverage changes

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