Fix flaky memory test

[JHolba]

Register only the memory allocations of the function we are trying to test.

• PR title captures the intent of the changes, and is fitting for release notes.
• Added appropriate release note label
• Commit history is consistent and clean, in line with the contribution guidelines.
• Make sure unit tests pass locally after every commit (git rebase -i main --exec 'pytest tests/ert/unit_tests -n logical -m "not integration_test"')

When applicable

• When there are user facing changes: Updated documentation
• New behavior or changes to existing untested code: Ensured that unit tests are added (See Ground Rules).
• Large PR: Prepare changes in small commits for more convenient review
• Bug fix: Add regression test for the bug
• Bug fix: Create Backport PR to latest release

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.37%. Comparing base (53fe7c0) to head (c0d51b3).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8812      +/-   ##
==========================================
+ Coverage   91.31%   91.37%   +0.05%     
==========================================
  Files         342      342              
  Lines       20985    20995      +10     
==========================================
+ Hits        19163    19184      +21     
+ Misses       1822     1811      -11     

Flag	Coverage Δ
cli-tests	39.60% <ø> (+0.02%)	⬆️
gui-tests	73.53% <ø> (+0.04%)	⬆️
performance-tests	50.07% <ø> (+<0.01%)	⬆️
unit-tests	79.85% <ø> (+0.06%)	⬆️

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[eivindjahren]

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