chore: add visual testing framework

[LeoMcA]

Relates to #1282

Tested that tests fail with a visual difference introduced: https://github.com/mdn/fred/actions/runs/26113567387/job/76796836889?pr=1588#step:13:64

This is a first step, things out of scope which will come in follow-up PRs:

• Converting the test cli to use commands, and merging the visual-report cli in: using a long string of options makes the command hard to use, and complicates implementation chore(tests): move from args to commands in cli #1590
• Optimising test running: either only running tests which generate screenshots as part of the baseline first run, or running tests against main to generate a baseline and using those in each PR; either way, only really necessary when our number of tests grows
• Uploading the visual diff: well worth doing, and probably the next step, but a little more complicated, and we have a decent alternative for now being able to download the artifact and view locally

[github-actions]

baed6c3 was deployed to: https://fred-pr1588.review.mdn.allizom.net/