Version 6.0.1+ ignores --silent flag

I've created a reproduction of this bug here: https://github.com/apellerano-pw/htmlbars-silent-repro

**Steps to reproduce**
1. have an ember app with some kind of deprecation output, e.g. use `{{#with}}` in Ember 3.28
2. use ember-cli-htmlbars 6.0.1 or higher
3. run `ember test --silent` (in the repro repo you can do `npm run repro`)

**Expected**

There is no build output, only test output

**Actual**

Deprecation output will be shown in addition to test output

**Workaround**

Pin your version of ember-cli-htmlbars to 6.0.0