Configuring addon test build

I have an addon that uses `async/await` in its `addon/` directory and its `tests/` directory.

I figured that since I'm testing on a modern headless Chrome, I could skip the transform for `async/await`. I tried

```js
// ember-cli-build.js
let app = new EmberAddon(defaults, {
babel: {
exclude: [
'transform-async-to-generator',
'transform-regenerator',
],
},
})
```

on the assumption that that would apply to both `addon/` and `tests/` **when building the dummy app**. It worked for `tests.js`, but not `vendor.js`. I still see `regeneratorRuntime.wrap` for each `async` method in `addon/`.

I'm _not_ trying to control what happens when my addon gets used in another app. That app knows more about its target browsers and should be able to determine what transforms are needed.

I can turn off the transforms for `addon/` if I configure `index.js`:
```js
// index.js
module.exports = {
name: '@my/addon',

babel: {
exclude: [
'transform-async-to-generator',
'transform-regenerator',
]
}
}
```

But that's going to affect any consuming app as well, which I don't want to do.