[test-runner] visual regression testing

[LarsDenBakker]

We should support visual regression testing with the test runner. Because we run tests in a real browser, it will be relatively easy to support this within the test runner. Most of the basic building blocks are already present.

Reference discussion: https://github.com/modernweb-dev/web/discussions/427

[daKmoR]

Additional Features (maybe via plugin) could be

• Open "Compare UI" in Browser to inspect images (similar link as code coverage?)
• Allow to "approve" a new image within the "Compare UI"
• Allow to "approve" a new image while running within the CLI menu?
• Save approved images not to git but to an external host
• Have plugins for suitable image hosters (find at least one free)
• Image Hoster Ideas
  • Google Drive
  • Amazon S3 or Amazon EFS
  • ImageShack
  • Artifactory (for corporations)
• Integrate with existing solutions
  • Percy
  • applitools

[LarsDenBakker]

Moved the discussion to https://github.com/modernweb-dev/web/discussions/427

[LarsDenBakker]

We now have a plugin which implements the first basic parts: https://github.com/modernweb-dev/web/tree/master/packages/test-runner-visual-regression