We run the external network tests on builders, but some of our
builders have less-than-ideal DNS connectivity. This change continues
to run the tests on all builders, but marks certain builders as flaky
(network-wise), and only validates their DNS results if they got DNS
results.

Change-Id: I826dc2a6f6da55add89ae9c6db892b3b2f7b526b
Reviewed-on: https://go-review.googlesource.com/22852Reviewed-by: Ian Lance Taylor <iant@golang.org>