`files(e)` logic seems to be incorrect for testing

The [`files(e)`](https://github.com/thefrontside/emberx-file-input/blob/master/addon/components/x-file-input.js#L59-L67) conditional appears to be in the wrong order for testing because `e.target.files` will be an empty `FileList` which is truthy so the `testingFiles` branches are never reached.

I changed the behaviour in an override component to the following which is working fine for me but I may have missed something:

```
files(e) {
return (e.originalEvent || e).testingFiles || e.target.files;
}
```