Some tests break under npm5

The `module-whitelist` test fixture injects two unpublished node modules directly into the node_modules directory before calling `npm install`. This is tolerated by npm 4. But it's an error in npm 5.

```
npm ERR! code E404
npm ERR! 404 Not Found: fake-addon@*
```

This problem is compounded by the fact that ember-cli-addon-tests [swallows exceptions](https://github.com/tomdale/ember-cli-addon-tests/issues/163) inside its app.run method, so you won't notice right away that npm is failing, and you get errors later in the test when modules are not found.