Improved the screenshots workflow.

[ngnpope]

Trac ticket number

N/A

Branch description

Makes the following improvements to the screenshots workflow:

• Removed summary and obsolete identifier (left over from earlier version of the implementation)
• Optimized images to reduce size by ~30%
• Changed to save images to <case>/<test-name>-<name>.png instead of <test-name>_<case>-<name>.png
• Added a top-level commit hash folder in the artifact archive, i.e. <commit-hash>/<case>/<test-name>-<name>.png
• Fail the workflow if the upload artifacts step doesn't upload any files

These changes are particularly useful when generating a series of screenshots and downloading for comparison, making it easier to find things.

Checklist

• This PR targets the main branch.
• The commit message is written in past tense, mentions the ticket number, and ends with a period.

[ngnpope]

👋🏻 @sarahboyce These were some things I thought could be improved that I spotted when I was reviewing #18080.

[sarahboyce]

Thank you for this!

[sarahboyce]

[Note that I have done ever so slight changes to the commits but no changes to the content, I am approving the first 3 commits which are a series of improvements to the screenshots workflow. I will pull these out and merge separately. The last two commits each require manual test runs that I plan to do later.]

[sarahboyce]

Thank you 👍