test: implement end-to-end tests

Add two basic end-to-end tests checking that the User model and
the authentication are correctly set up by the template.

/to @ritch please review

~~Note: the `npm install` step takes about 12 seconds on my machine. Instead of moving the tests to a different location, I'd like to implement a simple caching mechanism, so that `npm install` is run on the first run, and then a simple `ncp` is used instead. I'd rather make that change in a different pull request.~~

I have a feeling this will fail on ci.