This change deflakes TestDialerDualStackFDLeak, TestDialerDualStack,
TestResolve{TCP,UDP,IP}Addr by removing external dependencies.

Fixes #8764.

Change-Id: I5cca0a93776cf05652e0e6a4a4ff4af392ccb885
Reviewed-on: https://go-review.googlesource.com/8485Reviewed-by: Ian Lance Taylor <iant@golang.org>