[css-anchor-position] Move position-fallback-bounds tests

The position-fallback-bounds property is no longer part of the
specification. The implementation is still present behind a separate
flag. Move the tests to wpt_internal.

Bug: 329301692
Change-Id: I30ec46f8eacdd1bc17fdb016c6049cd0e398c8ab
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5422810


Commit-Queue: Rune Lillesveen <futhark@chromium.org>
Reviewed-by: Anders Hartvoll Ruud <andruud@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1282780}