[line-clamp] Move (-webkit-)line-clamp WPTs to a separate folder
This also updates the css-line-clamp virtual test suite to run only
the tests in that folder.
Bug: 40336192
Change-Id: I60e01a382208dccfab106292c498eb5209b9867f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5629255
Reviewed-by: Ian Kilpatrick ikilpatrick@chromium.org
Commit-Queue: Andreu Botella abotella@igalia.com
Cr-Commit-Position: refs/heads/main@{#1314555}