Ignore cartopy DownloadWarning in pytest

[jfrost-mo]

As CI starts from a fresh slate each time we won't yet have the resources downloaded. We do cache them, but caches are only per-branch, and thus we still need to download on the first run per branch.

Also removed the dateutil warning filter, as that issue has been fixed.

Contribution checklist

Aim to have all relevant checks ticked off before merging. See the developer's guide for more detail.

• [ ] Documentation has been updated to reflect change.
• [ ] New code has tests, and affected old tests have been updated.
• [ ] All tests and CI checks pass.
• [x] Ensured the pull request title is descriptive.
• [ ] Conda lock files have been updated if dependencies have changed.
• [ ] Attributed any Generative AI, such as GitHub Copilot, used in this PR.
• [ ] Marked the PR as ready to review.

[github-actions[bot]]

[jfrost-mo]

Running tests after manually purging the caches to validate the change.

[jfrost-mo]

Looking good: https://github.com/MetOffice/CSET/actions/runs/9714912375/job/26815224988?pr=716#step:6:242