[css] Add (failing) test for ::part(part1 part2 ...)

This was added to the draft spec in

https://github.com/w3c/csswg-drafts/commit/7f2f64b235d4bc14eaef2cddff986b8af53964fa


Bug: 955897
Change-Id: I9df61e705623a84d8084059790b80d8f43a37f84
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1580688
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Commit-Queue: Fergal Daly <fergal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#653961}
2 files changed