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.
Oh, I didn't realize it was replacing HTML contents, interesting. OK, should be good. Will pull down and test on Windows to make sure.