Apply options to `addon-test-support` tree of an Engine

It seems like the Babel options provided to an Engine are not applied to the `addon-test-support` tree.

I have the following configuration in my Engine's `index.js`:

```javascript
module.exports = EngineAddon.extend({
// ...

babel: {
plugins: [
['transform-object-rest-spread', { useBuiltIns: true }]
]
},

// ...
});
```

And am still having the following Babel compilation issue:

```text
dummy/tests/pages/tracker.js: Unexpected token (23:4)

21 | taskLists: collection(taskListSelector, taskListPageObject),
22 | taskCreationModal: {
> 23 | ...taskFormModalPageObject
| ^
24 | },
25 |
26 | createTracker: clickable('[data-test-create-tracker]')
```