Bug 1014400 - [Test Agent] Specify mocha options without patching test-agent itself

how about:

```javascript
var result = target || {};
var objs = Array.slice(arguments, 1);
return objs.reduce(function(result, obj) {
  for (var key in obj) {
    if (obj.hasOwnProperty(key)) {
      result[key] = obj[key];
    }
  }
}, result);
```

Make it generic enough that we can reuse it elsewhere if needed.