Upload artifacts showing diff images on test failure

[weiji14]

Description of proposed changes

When our unit tests fail, it is useful sometimes to see what went wrong. Specifically, tests with pytest-mpl.mpl_image_compare and check_figures_equal (#555) will return an image diff on test failures in the 'tmp-test-dir-with-unique-name' directory, and we can upload those files as a Github artifact for inspection.

Uses the Upload Artifact Github Action.

Originally suggested at #471 (comment)

Fixes #

Reminders

• Run make format and make check to make sure the code follows the style guide.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst.
• Write detailed docstrings for all functions/methods.
• If adding new functionality, add an example to docstrings or tutorials.

Notes

• You can write /format in the first line of a comment to lint the code automatically

[seisman]

Looks good, just a few more comments.