Fix ambiguous find_unsmry()

[berland]

Issue Resolves #9277

Approach Solve old bug that old code tucked under the carpet.

To reproduce this problem you need to run Eclipse with more than 1 CPU and also leave a similar looking UNSMRY-file to the runpath.

• [x] PR title captures the intent of the changes, and is fitting for release notes.
• [x] Added appropriate release note label
• [x] Commit history is consistent and clean, in line with the contribution guidelines.
• [x] 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
• [x] Bug fix: Add regression test for the bug
• [ ] Bug fix: Create Backport PR to latest release

[berland]

Seems like the new test is flaky:

FAILED tests/ert/unit_tests/resources/test_ecl_run_new_config.py::test_find_unsmry[paths_to_touch11-SPE1-ValueError] - Failed: DID NOT RAISE <class 'ValueError'>

https://github.com/equinor/ert/actions/runs/11955842976/job/33364180931?pr=9300

Oh no, case insensitive filesystem on Mac.