net/dns/resolver: add test for upstream resolvers reached via netstack (#20779)

sendTCP dials through tsdial.Dialer and so honors UseNetstackForIP, but
sendUDP always uses a host-stack socket. In userspace networking mode
there is no route to the tailnet, so a split-DNS query to a tailnet
resolver only succeeds after falling back to TCP. The two subtests
differ only in transport, isolating that gap; the UDP one is skipped
pending a fix.

Updates #20314

Signed-off-by: Brendan Creane <bcreane@gmail.com>