This change consolidates test helpers that test platform capabilities.
testNetwork, testAddress and testListenArgs report whether given
ariguments are testable on the current platform configuration to
mitigate to receive weird test results.

Change-Id: Ie1ed568a1f9cc50f3155945ea01562904bc2c389
Reviewed-on: https://go-review.googlesource.com/8076Reviewed-by: Ian Lance Taylor <iant@golang.org>