unix: fix TestSCMCredentials

Port http://golang.org/cl/9714 from the main syscall package.

Change-Id: I85e3f119777b0e1f52dfd10365fd30c900120dc2
Reviewed-on: https://go-review.googlesource.com/9961
Reviewed-by: Minux Ma <minux@golang.org>

unix/creds_test.go

@@ -58,7 +58,13 @@ func TestSCMCredentials(t *testing.T) {
 		ucred.Gid = 0
 		oob := unix.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)
 		}
 	}