test: introduce a broadcast helper

Introduce a helper - very similar to `common.localhostIPv4` that allows test
runner to override a broadcast address. This is required in certain
scenarios - like FreeBSD jails - since networking acts a bit different.

Refs: nodejs#2472

test/common.js

@@ -68,6 +68,7 @@ exports.tmpDir = path.join(exports.testDir, exports.tmpDirName);
 var opensslCli = null;
 var inFreeBSDJail = null;
 var localhostIPv4 = null;
+var broadcastIPv4 = null;
 
 Object.defineProperty(exports, 'inFreeBSDJail', {
   get: function() {
@@ -107,6 +108,27 @@ Object.defineProperty(exports, 'localhostIPv4', {
   }
 });
 
+Object.defineProperty(exports, 'broadcastIPv4', {
+
+  get: function() {
+    if (broadcastIPv4 !== null) return broadcastIPv4;
+
+    if (exports.inFreeBSDJail) {
+      if (process.env.BROADCAST) {
+        broadcastIPv4 = process.env.BROADCAST;
+      } else {
+        // shorter error than above since you'd probably be seeing both
+        console.error('In a FreeBSD jail. Pass broadcast as \'BROADCAST=\' ' +
+                      'to avoid test failures.');
+      }
+    }
+
+    if (broadcastIPv4 === null) broadcastIPv4 = '255.255.255.255';
+
+    return broadcastIPv4;
+  }
+});
+
 // opensslCli defined lazily to reduce overhead of spawnSync
 Object.defineProperty(exports, 'opensslCli', {get: function() {
   if (opensslCli !== null) return opensslCli;

test/internet/test-dgram-broadcast-multi-process.js

@@ -6,7 +6,7 @@ var common = require('../common'),
     networkInterfaces = require('os').networkInterfaces(),
     Buffer = require('buffer').Buffer,
     fork = require('child_process').fork,
-    LOCAL_BROADCAST_HOST = '255.255.255.255',
+    LOCAL_BROADCAST_HOST = common.broadcastIPv4,
     TIMEOUT = common.platformTimeout(5000),
     messages = [
       new Buffer('First message to send'),