Fix hfe no malloc size unit

[ranshid]

test_vest uses a mock_defrag function in order to simulate defrag flow.
In the scenario of no system malloc_size we need to use zmalloc_usable_size in order to prevent stepping over the "to-be-replaced" buffer

will partially fix: #2435

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.52%. Comparing base (db11e41) to head (12f1c98).
⚠️ Report is 1 commits behind head on unstable.

Additional details and impacted files

@@             Coverage Diff              @@
##           unstable    #2436      +/-   ##
============================================
- Coverage     71.52%   71.52%   -0.01%     
============================================
  Files           125      125              
  Lines         69213    69213              
============================================
- Hits          49505    49503       -2     
- Misses        19708    19710       +2     

see 17 files with indirect coverage changes

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