[anchor] Add WPTs for transitions triggered by anchor changes

This CL adds coverage which verifies that transitions are triggered in the following cases:

• When the anchor()/anchor-size() remains the same, but the anchor element moves around, changing the result of the anchor functions.
• When the element referenced by a name changes.
• When the anchor attribute is changed.
• When the default anchor (position-anchor) changes.

Bug: 40943051 Change-Id: I3f65a4650ce696f06be5cc71f4e2d68364d1fb10 Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5419979 Commit-Queue: Anders Hartvoll Ruud andruud@chromium.org Reviewed-by: Rune Lillesveen futhark@chromium.org Cr-Commit-Position: refs/heads/main@{#1283083}