crypto/x509: skip SystemRootsError test on Windows

On Windows, crypto/x509 passes through to Windows's CryptoAPI
to verify certificate chains. This method can't produce a
SystemRootsError, so make sure we always skip the test on
Windows.

This is needed because testVerify is called in both
TestGoVerify and TestSystemVerify on Windows - one is for
testing the Go verifier, the other one is for testing the
CryptoAPI verifier. The orignal CL tried to sidestep
this issue by setting systemSkip to true, but that only
affected TestSystemVerify.

R=golang-dev, agl, snaury, minux.ma
CC=golang-dev
https://codereview.appspot.com/7185043

Committer: Shenghou Ma <minux.ma@gmail.com>