WPT: Move focus-preserve.html out of /tentative status

This CL is a follow-up of https://crrev.com/c/6335812.

R=nrosenthal

Bug: 40150299
Change-Id: Id62d741f2b10cfa45b41eb1e4f54e739ad16f62d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6405353
Reviewed-by: Noam Rosenthal <[email protected]>
Commit-Queue: Dominic Farolino <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1439024}

Author: domfarolino