src: avoid copy when creating Blob
Cherry-picked and squashed two commits from @jasnell's #44325 in order to make the QUIC PR simpler to review (with his permission). This change makes sense even without the QUIC PR.
Do not update/delete: Banner broadcast message test data
Do not update/delete: Notification broadcast message test data
Cherry-picked and squashed two commits from @jasnell's #44325 in order to make the QUIC PR simpler to review (with his permission). This change makes sense even without the QUIC PR.