update test/double_end.js to use path.join for cross-platform compatibility see: #314 (comment)

Author: nelsonic

test/double_end.js

@@ -1,10 +1,11 @@
 var test = require('tap').test;
+var path = require('path');
 var concat = require('concat-stream');
 var spawn = require('child_process').spawn;
 
 test(function (t) {
     t.plan(2);
-    var ps = spawn(process.execPath, [ __dirname + '/double_end/double.js' ]);
+    var ps = spawn(process.execPath, [path.join(__dirname, 'double_end', 'double.js')]);
     ps.on('exit', function (code) {
         t.equal(code, 1);
     });