Revert "net: add missing error check in test"

This reverts commit ec521467.

Reson for revert: The test cases using slowDst4 and slowDst6 are
fragile. We need to find out a better approach to the trick on the IP
routeability.

Change-Id: I544453886e809d1c7b339673d8f1d5bdef357147
Reviewed-on: https://go-review.googlesource.com/c/163919


Reviewed-by: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>