[feature] make snapshot tests less noisy

[elliottneilclark]

When building a larger component, smaller CSS changes are less important. For example, going from mx-2 to mx-4 in the left column isn't essential. We should add a way for the deep comparison of nodes to optionally ignore some CSS.

The idea is to add an option to SnapshotCase that causes compare_attrs to dive into the class attribute, splitting the classes being added and ignoring margin, padding, class ordering, and other differences that don't matter.

Tests for small components (links or button) will want the current behavior.
Tests for aggregate components like table that are testing larger functionality can ignore more relying on tests of smaller components.