[scoped-registry] Test changes

This patch:

1. Moves the test directory from wpt_internal to wpt, and change
tests to tentative, so that tests are public to non-Blink developers.

2. Adds a virtual test suite to verify that this experimental feature
is not leaked to the web. (Also added a DCHECK to code to ensure this)

Bug: 1304439
Change-Id: Ib027253b17d47157678105595b19cd9b6ee0c530
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3924394
Reviewed-by: Mason Freed <masonf@chromium.org>
Auto-Submit: Xiaocheng Hu <xiaochengh@chromium.org>
Commit-Queue: Mason Freed <masonf@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1053665}