net: allow TestDialListenerAddr without an external network

net: allow TestDialListenerAddr without an external network

As of CL 482163, this test now works when only a loopback network is
available.

Updates #59497.

Change-Id: I32be4b74bbc663eb109763ea19d79f22c63f50ae
Reviewed-on: https://go-review.googlesource.com/c/go/+/483696
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Bypass: Bryan Mills <bcmills@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>