Add screenshot variants

[gregberge]

As a user, I want to mark multiple screenshot variations as valid for a diff to reduce the flaky tests.

This feature will be done after screenshots review.

[ryota-murakami]

Awesome, this is so effective against animation/transition!

[moritzuehling]

I'd love to see that one, flaky tests that are in one of two states are the most frequent detected change.