net: rename dialgoogle_test.go to external_test.go

In the followup changes, tests that require external facilities such as
DNS servers and RRs will move into external_test.go.

Change-Id: Ib460b0c51961159830357652dbf5430e1ba01514
Reviewed-on: https://go-review.googlesource.com/9461Reviewed-by: Dave Cheney <dave@cheney.net>