add simple tests

extra_modules/xor.js

@@ -0,0 +1,27 @@
+function XOR_hex(a, b) {
+    var res = "",
+        i = a.length,
+        j = b.length;
+    while (i-->0 && j-->0)
+        res = (parseInt(a.charAt(i), 16) ^ parseInt(b.charAt(j), 16)).toString(16) + res;
+    return res;
+}
+
+function hexEncode(a){
+    let hex;
+
+    let result = "";
+    for (let i=0; i<a.length; i++) {
+        hex = a.charCodeAt(i).toString(16);
+        result += ("000"+hex).slice(-4);
+    }
+
+    return result
+}
+
+function xor(a,b) {
+    return XOR_hex(hexEncode(a),hexEncode(b)).toString("hex")
+}
+
+module.exports = xor
+