Before sending a fix for #18991, this adds an integration test that
locks in that the proxymap WhoIs code works with two nodes running as
different users, with the second node running a localhost service and
able to use its local tailscaled to identify a Tailscale connection
from the other tailscaled.
Updates #18991
Change-Id: I6fbb0810204d77d2ac558f0cc786b73e3248d031
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
4c91f90776