#ember-testing-container is alway empty and shows no preview.

Hey!

I came across this issue working with my application tests, when they are being executed, there is no preview shown for the application.

Maybe the issue is very similar to the issue outlined over [here](https://github.com/emberjs/ember-qunit/issues/764).

Any help would be appreciated, thanks!

`package.json`
```js
"ember-mocha": "^0.16.2",
"ember-cli": "~3.26.1",
...
```

Start with the versions listed in package.json and reproduce the application test behavior using ember-mocha with ember-cli ~3.26.1. Compare the result with the linked ember-qunit issue and inspect how the test container is populated. Done means the application preview appears in #ember-testing-container during test execution.

Written by the indexing model from the issue text.