net: avoid opening an external port in TestDialListenerAddr

net: avoid opening an external port in TestDialListenerAddr

The behavior in #18806 can be tested with a localhost-only port,
provided that we're willing to assume what format the listener would
report for an external dual-stack port.

Fixes #59497.

Change-Id: I171fb03eb46aee8e85480e04626a23f4f3b923e9
Reviewed-on: https://go-review.googlesource.com/c/go/+/482163
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Bryan Mills <bcmills@google.com>