First functional tests for scrollbar-gutter #24553
Tests for the values
The basic test works by comparing the size of the container and contained elements, which should be the same unless a scrollbar gutter is being shown.
Tests for the values "auto", "stable" and "always" of the scrollbar-gutter property with classic scrollbars. Spec: https://drafts.csswg.org/css-overflow-4/#scrollbar-gutter-property