Deprecation warning violates `ember test --silent` behavior

According to the help for `ember test`:

```
--silent (Boolean) (Default: false) Suppress any output except for the test report
```

But the deprecation warning `DEPRECATION: Overriding init without calling this._super is deprecated. Please call this._super()` busts through that flag, breaking an automated CI workflow (because the CI output now includes these deprecation warnings instead of being only the test results).