Exclude files while testing with Ember-CLI

I am trying to run my tests using Ember-CLI - Testem.
Ember-CLI uses tests/index.html and not the usual testem.json config for testing.

Is there any way I can exclude certain files from being built into app.js?

Usecase : I have some js files where I inject some dependencies. These dependencies are different during the testing environment. I would like to ignore these files and inject the dependencies from my test-injectors.