Fix flaky warning test

[eivindjahren]

cannot guarantee that there is no warning, but the test is specifically about ConfigWarning.

Resolves #8990

• 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 91.55%. Comparing base (e5a183d) to head (3b992cd).
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8992      +/-   ##
==========================================
+ Coverage   91.49%   91.55%   +0.06%     
==========================================
  Files         347      347              
  Lines       21397    21397              
==========================================
+ Hits        19577    19591      +14     
+ Misses       1820     1806      -14     

Flag	Coverage Δ
cli-tests	39.61% <ø> (+0.20%)	⬆️
gui-tests	73.58% <ø> (+0.02%)	⬆️
performance-tests	50.19% <ø> (ø)
unit-tests	80.10% <ø> (-0.02%)	⬇️

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[andreas-el]

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