FIX: Assets for the theme tests page are not compressed (PR #13736)

A couple of weeks we made a change that skipped compressing assets used by the theme qunit page: DEV: Skip compressing theme test files by pmusaraj · Pull Request #13619 · discourse/discourse · GitHub. This is a follow-up PR to stop the application helper from generating the assets for the theme qunit page with .br or .gzip extensions when a site uses S3 as a CDN.