Version 6.0.1+ ignores --silent flag

[apellerano-pw]

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

[apellerano-pw]

Here's the diff between 6.0.1 and 6.0.0.

v6.0.0...v6.0.1

It's likely this PR but I don't understand Ember's build process enough to say why

#739