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

from an object point of view, we need a copy of setup here, instead of modifying the common instance.

So, if we have an "extend"-like method, we can do:
```javascript
var setup = extend({}, self.setup, { reporter: self.getReporter(box) });
```

without extend:
```javascript
var setup = {};
for (var k in self.setup) {
  setup[k] = self.setup[k];
}
setup.reporter = self.getReporter(box);
```

Maybe you can make this a new method?