Enable installation tests

This pull request makes it possible for the extension installation tests to run by enabling the deletion of directories via Node.

Note: to keep this pull request clean, I have not yet committed the node_modules used by the new TestingDomain. To run the ExtensionInstallation tests here, you'll need to:

```
cd test/node
npm install
```

I wanted to get this committed so that a couple of basic extension installation tests can run end-to-end. Ultimately, I'd like to see the TestingDomain be able to run jasmine-node tests with the results reported in the client side test runner.