Add test template generation.

This is not quite finished, but I figured I'd might as well get a PR going to get some feedback.

When we are ready to merge I can just squash everything down, but for now it is easier to pick through the commits if they are smaller.

The main issue that I have is how to get the module path of the object under test. In `ember-tools` this was provided by the `{{modulePath}}` variable (see [here](https://github.com/rpflorence/ember-tools/blob/tests/src/templates/generate/tests/controller.test.js.hbs#L1)), but with `loom` it is a bit more generalized. Suggestions?