Change min realizations warning test from cli to unit

[eivindjahren]

Changes one simple cli test to a unit test. Avoids ~1 minute worth of run time.

• 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

[codspeed-hq]

CodSpeed Performance Report

Merging #9747 will not alter performance

_{Comparing eivindjahren:speed_up_cli_test (4bdc45d) with main (b6edc3b)}

Summary

✅ 24 untouched benchmarks

[berland]

yay