DEV: Support for running theme test with Ember CLI (third attempt) (PR #15571)

The second attempt fixed issues with smoke test.

This one makes sure minification only happens in production mode.

GitHub

For reviewers, the difference between the previous PR is this ~3-line diff: Comparing 2278c7f82dd8e7f24dc6dc66bc6fea02e598c6d0..ad2a79fa389940edb94322e730f59fadb038829b · discourse/discourse · GitHub