testing: be explicit about use of b.N

R=golang-dev, minux.ma
CC=golang-dev
https://codereview.appspot.com/7314071
crypto/x509: disable SCG test with system validation.

On Windows, CryptoAPI is finding an alternative validation path. Since
this is a little non-deterministic, this change disables that test
when using system validation.

R=golang-dev
CC=golang-dev
https://codereview.appspot.com/7313068