This command allows observing whether a given dialer ("SystemDial",
"UserDial", etc.) will successfully obtain a connection to a provided
host, from inside tailscaled itself. This is intended to help debug a
variety of issues from subnet routers to split DNS setups.
Updates #9619
Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: Ie01ebb5469d3e287eac633ff656783960f697b84