crypto/cipher: disable broken js/wasm test from nodejs v8 to v12 upgrade

Updates #31812

Change-Id: Id9898f89205c116009e25033afb5b9026594e80f
Reviewed-on: https://go-review.googlesource.com/c/go/+/175099Reviewed-by: Andrew Bonventre <andybons@golang.org>

src/crypto/cipher/xor_test.go

@@ -9,11 +9,16 @@ import (
 	"crypto/cipher"
 	"crypto/rand"
 	"fmt"
+	"internal/testenv"
 	"io"
+	"runtime"
 	"testing"
 )
 
 func TestXOR(t *testing.T) {
+	if runtime.GOOS == "js" {
+		testenv.SkipFlaky(t, 31812)
+	}
 	for j := 1; j <= 1024; j++ {
 		for alignP := 0; alignP < 2; alignP++ {
 			for alignQ := 0; alignQ < 2; alignQ++ {