Small fixes in tests and docs of misfit_preprocessor

[eivindjahren]

Quick cleaning of tests/docs before tackling #9301

• 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 91.84%. Comparing base (b903bb0) to head (7a6c7fc).
Report is 7 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9471      +/-   ##
==========================================
- Coverage   91.86%   91.84%   -0.02%     
==========================================
  Files         433      433              
  Lines       26654    26686      +32     
==========================================
+ Hits        24486    24511      +25     
- Misses       2168     2175       +7     

Flag	Coverage Δ
cli-tests	39.40% <ø> (-0.04%)	⬇️
everest-models-test	34.80% <ø> (-0.02%)	⬇️
gui-tests	72.28% <ø> (-0.11%)	⬇️
integration-test	37.06% <ø> (-0.02%)	⬇️
performance-tests	52.01% <ø> (-0.06%)	⬇️
test	40.73% <ø> (-0.02%)	⬇️
unit-tests	74.02% <ø> (+<0.01%)	⬆️

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