Make tests available offline

See #817.

To execute the tests the repo has to be cloned with:
```bash
git clone --recursive git://github.com/tmpvar/jsdom.git
```
or after the initial clone:
```bash
git submodule update --init --recursive
```

This is just a very simple rework to make the simple tests available offline. The more complex ones which require specific server or /etc/hosts behaviour need to be handled differently.
But on this line and the below one, it should be e.g. `path.resolve(__dirname, 'tests/resources/')` and `path.resolve(__dirname, 'tests', url)`, right?