Mark auctionReportBuyers debug mode experimental

Also fixes feature checking in validation code, given that simply
marking a field as RuntimeEnabled does not affect certain checks. Also
adds the newly disabled feature to virtual test suites to keep WPTs
passing.

Bug: 329032156
Change-Id: I8bcb20e26df1feb46f542f2a64d7012110be5ac9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5359415
Commit-Queue: Alex Turner <alexmt@chromium.org>
Reviewed-by: Qingxin Wu <qingxinwu@google.com>
Reviewed-by: Ian Kilpatrick <ikilpatrick@chromium.org>
Auto-Submit: Alex Turner <alexmt@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1271043}