This change adds testHookLookIP to enable to inject DNS name to IP
address mappings for Happ{y,yish,ier} Eyeballs dial testing.

Change-Id: I8ac04a594e1e2bd77909528df0552889914a7790
Reviewed-on: https://go-review.googlesource.com/8399Reviewed-by: Ian Lance Taylor <iant@golang.org>