syscall: fix TestSCMCredentials

Fixes #10703.

Change-Id: I55c0f07625a0847fb27defa9891af6db6eb21f82
Reviewed-on: https://go-review.googlesource.com/9714Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/syscall/creds_test.go

@@ -56,7 +56,13 @@ func TestSCMCredentials(t *testing.T) {
 		ucred.Gid = 0
 		oob := syscall.UnixCredentials(&ucred)
 		_, _, err := cli.(*net.UnixConn).WriteMsgUnix(nil, oob, nil)
-		if err.(*net.OpError).Err != syscall.EPERM {
+		if op, ok := err.(*net.OpError); ok {
+			err = op.Err
+		}
+		if sys, ok := err.(*os.SyscallError); ok {
+			err = sys.Err
+		}
+		if err != syscall.EPERM {
 			t.Fatalf("WriteMsgUnix failed with %v, want EPERM", err)
 		}
 	}