[wpt] Make justify-items-anonymous non-tentative.

CSSWG resolution on Option (1) - the behaviour we changed to in:
https://chromium-review.googlesource.com/c/chromium/src/+/6512658

w3c/csswg-drafts#11461 (comment)

Make test added in that patch non-tentative.

Fixed: 417734173
Change-Id: I4af723d2b39764c77507be038c31214da42f1c69
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6548615
Commit-Queue: Morten Stenshorne <[email protected]>
Reviewed-by: Morten Stenshorne <[email protected]>
Auto-Submit: Ian Kilpatrick <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1460363}

Author: bfgeek