Rename popover anchor positioning tests back to .tentative

These use non-standard APIs, mostly the `anchor` attribute but
also some anchor positioning itself. As such, they should be
.tentative.

Bug: 1309178
Change-Id: Ief9c1f6375bc234b929b9301980d4be51de0b9c2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5026613
Commit-Queue: Mason Freed <[email protected]>
Auto-Submit: Mason Freed <[email protected]>
Reviewed-by: Di Zhang <[email protected]>
Commit-Queue: Di Zhang <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1223830}