Run cli tests not in parallell

[larsevj]

This appears to be consistently faster than running them in parallel on the github hosted runners.
Might be due to different tests fighting for the same resources and throttling the performance.
Also avoids the heat equation running twice in some cases where tests were distributed poorly (could have been avoided with xdist-group as well)

• 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