Taildrop has never had an end-to-end test since it was introduced.
This adds a basic one.
It caught two recent refactoring bugs & one from 2022 (0f7da5c7dc0).
This is prep for moving the rest of Taildrop out of LocalBackend, so
we can do more refactorings with some confidence.
Updates #15812
Change-Id: I6182e49c5641238af0bfdd9fea1ef0420c112738
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>