test: fix test-buffer-zero-fill-cli to be effective

[ChALkeR]

The test was useless for testing --zero-fill-buffers flag, which is what it was supposed to test.

1. Buffer.allocUnsafeSlow(20) is zero-filled regardless of the flag because it's from heap (<=64), not an alloc

2. 50 first calls to Buffer.allocUnsafe(20) were zero-filled regardless of the flag because they all come from the pool which was in the snapshot, and no real runtime alloc was made.

This PR fixes the test, so it now fails without the flag (as expected).

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/70091/

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/70092/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.55%. Comparing base (dbe45b7) to head (c69267a).
⚠️ Report is 67 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #60623      +/-   ##
==========================================
+ Coverage   88.54%   88.55%   +0.01%     
==========================================
  Files         704      704              
  Lines      207843   208097     +254     
  Branches    40044    40079      +35     
==========================================
+ Hits       184028   184283     +255     
+ Misses      15858    15833      -25     
- Partials     7957     7981      +24     

see 62 files with indirect coverage changes

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[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/70097/