Most of the package:palette_generator canvaskit web tests fail in CI, but not locally.

Errors seem to point to some sort of wrong decoding of the image colors in CI for some reason (2 colors detected when the image has only one, weird color tones when extracting a palette...)

In order to configure CI to run web tests using canvaskit (the new default for web applications), and since package:palette_generator seems to work normally outside of CI, web tests for package:palette_generator were disabled in this PR.

This issue is a reminder to:

Diagnose, and re-enable web platform tests for palette_generator.