[wpt] upgrade intersection-observer tests to non-tentative
These are supported cross vendor:
https://wpt.fyi/results/intersection-observer?label=experimental&label=master&aligned&q=tentative
Bug: 3534256
Change-Id: Ib15f706a7d5be6624db1e0073473d984b3317641
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5713670
Commit-Queue: Ari Chivukula arichiv@chromium.org
Commit-Queue: Steven Bingler bingler@chromium.org
Auto-Submit: Ari Chivukula arichiv@chromium.org
Reviewed-by: Steven Bingler bingler@chromium.org
Cr-Commit-Position: refs/heads/main@{#1328217}