crypto/x509: disable sha2 test with system APIs

Fixes #11730.

Change-Id: I5bc60779a87dc07899dd70659a830996bf7812ca
Reviewed-on: https://go-review.googlesource.com/12527
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/crypto/x509/verify_test.go

@@ -221,6 +221,10 @@ var verifyTests = []verifyTest{
 		currentTime:   1397502195,
 		dnsName:       "api.moip.com.br",
 
+		// CryptoAPI can find alternative validation paths so we don't
+		// perform this test with system validation.
+		systemSkip: true,
+
 		sha2: true,
 		expectedChains: [][]string{
 			{