[css-anchor-position] Add test for anchor-center with scroll

anchor-center positioning should have scroll adjustment when the anchor
scrolls, but the anchored doesn't.

Bug: 40277616
Change-Id: Ic5783cb60c4cc2dbc7801340a3cd393bd1285d7c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5425172


Commit-Queue: Rune Lillesveen <futhark@chromium.org>
Reviewed-by: Anders Hartvoll Ruud <andruud@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1283890}