Add tests for flow vs OMP_NUM_THREADS

[berland]

This serves as documentation and verification of the current interplay between ert, the plugin configuration of ert and the flowrun wrapper, and the flow binary itself.

Issue
Relates to #8668

• 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 tests/everest -n auto --hypothesis-profile=fast -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 #9834 will not alter performance

_{Comparing berland:flow_omp_num_threads_testing (9de4aac) with main (15109bb)}

Summary

✅ 24 untouched benchmarks