[wpt] upgrade loading tests to non-tentative
These are supported cross vendor:
https://wpt.fyi/results/loading?label=experimental&label=master&aligned&q=tentative
Bug: 3534256
Change-Id: I0933af64e7a9a7575cbb4154b1d01172dab05bf1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5713456
Reviewed-by: Steven Bingler bingler@chromium.org
Commit-Queue: Steven Bingler bingler@chromium.org
Commit-Queue: Ari Chivukula arichiv@chromium.org
Auto-Submit: Ari Chivukula arichiv@chromium.org
Cr-Commit-Position: refs/heads/main@{#1328220}