Update tests and turn on image comparison testing in CI

[kafitzgerald]

PR Summary

Updates tests to avoid hitting the seaborn import issue from xarray (see #256) and turns on image comparison testing in CI.

Doesn't alter our geocat-viz test coverage, but does avoid hitting the bug from upstream without requiring seaborn.

Closes #256
Closes #259

PR Checklist

General

• Make an issue if one doesn't already exist
• Link the issue this PR resolves by adding closes #XXX to the PR description where XXX is the number of the issue.
• Add a brief summary of changes to docs/release-notes.rst in a relevant section for the next unreleased release. Possible sections include: Documentation, New Features, Bug Fixes, Internal Changes, Breaking Changes/Deprecated
• Add appropriate labels to this PR
• Make your changes in a forked repository rather than directly in this repo
• Open this PR as a draft if it is not ready for review
• Convert this PR from a draft to a full PR before requesting reviewers
• Passes precommit. To set up on your local, run pre-commit install from the top level of the repository. To manually run pre-commits, use pre-commit run --all-files and re-add any changed files before committing again and pushing.

[kafitzgerald]

The only thing failing now is the link check which is addressed in #260.

[jukent]

Great. I'm a little surprised that this is slightly more involved than in NCAR/geocat-examples#628

[kafitzgerald]

Great. I'm a little surprised that this is slightly more involved than in NCAR/geocat-examples#628

Yeah, there's a bit more info in the associated issues, but in short the way these projects are impacted is different. In geocat-examples, our examples leverage functionality that triggers the bug so I opted to adjust the environment file and disallow the impacted version. For geocat-viz, we can avoid the bug altogether by adapting our tests without reducing test coverage so I opted to do that instead.