Add a pre-js script for providing /dev/urandom to tests in Javascript.

Not plugged to the build system yet.

test/default/pre.js

@@ -0,0 +1,33 @@
+try {
+    this['Module'] = Module;
+    Module.test;
+} catch(e) {
+    this['Module'] = Module = {};
+}
+Module['preRun'] = Module['preRun'] || [];
+Module['preRun'].push(function(){
+    var randombyte = null;    
+    try {
+        function randombyte_standard() {
+            var buf = new Int8Array(1);            
+            window.crypto.getRandomValues(buf);
+            return buf[0];
+        }
+        randombyte_standard();
+        randombyte = randombyte_standard;
+    } catch (e) {
+        try {
+            var crypto = require('crypto');
+            function randombyte_node() {
+                return crypto.randomBytes(1)[0];
+            }
+            randombyte_node();
+            randombyte = randombyte_node;
+        } catch(e) { }
+    }
+    FS.init();
+    var devFolder = FS.findObject('/dev') ||
+        Module['FS_createFolder']('/', 'dev', true, true);    
+    Module['FS_createDevice'](devFolder, 'random', randombyte);
+    Module['FS_createDevice'](devFolder, 'urandom', randombyte);    
+});