crypto/x509: update tests because Windows removed the Verisign root.

The root update on 3/11/2014 removed the Verisign root cert that the Go
tests use. This only affects the 'TestSystemVerify' test in
crypto/x509.

Fixes #7523.

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/80000044