crypto/x509: ignore 5 phantom 1024-bit roots in TestSystemRoots

On macOS 10.11, but not 10.10 and 10.12, the C API returns 5 old root
CAs which are not in SystemRootCertificates.keychain (but seem to be in
X509Anchors and maybe SystemCACertificates.keychain, along with many
others that the C API does not return). They all are moribund 1024-bit
roots which are now gone from the Apple store.

Since we can't seem to find a way to make the no-cgo code see them,
ignore them rather than skipping the test.

Fixes #21416

Change-Id: I24ff0461f71cec953b888a60b05b99bc37dad2ed
Reviewed-on: https://go-review.googlesource.com/c/156329Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/crypto/x509/root_darwin_test.go

@@ -5,6 +5,7 @@
 package x509
 
 import (
+	"crypto/rsa"
 	"os"
 	"os/exec"
 	"path/filepath"
@@ -104,6 +105,14 @@ func TestSystemRoots(t *testing.T) {
 			continue
 		}
 
+		// On 10.11 there are five unexplained roots that only show up from the
+		// C API. They have in common the fact that they are old, 1024-bit
+		// certificates. It's arguably better to ignore them anyway.
+		if key, ok := c.PublicKey.(*rsa.PublicKey); ok && key.N.BitLen() == 1024 {
+			t.Logf("1024-bit certificate only present in cgo pool (acceptable): %v", c.Subject)
+			continue
+		}
+
 		t.Errorf("certificate only present in cgo pool: %v", c.Subject)
 	}