This change makes TestDualStack{TCP,UDP}Listener work more properly by
attempting to book an available service port before testing.

Also simplifies error messages in tests.

Fixes #5001.

Change-Id: If13b0d0039878c9bd32061a0440664e4fa7abaf7
Reviewed-on: https://go-review.googlesource.com/9661Reviewed-by: Ian Lance Taylor <iant@golang.org>