updater: integration test for `needsadmin=prefers`

Bug: 40278378
Change-Id: Ie2a8dfa64663557445b9115919d1db57613653e8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5417397


Auto-Submit: S Ganesh <ganesh@chromium.org>
Reviewed-by: Xiaoling Bao <xiaolingbao@chromium.org>
Commit-Queue: S Ganesh <ganesh@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1281941}