How to test that CSS is being included by an addon

[rwjblue]

A few things that seem like obvious fixes we need to do (based on this thread and the other one that @lennyburdette started a little while back), and I think @lennyburdette listed them nicely in:

@lennyburdette

I think it’d be great to:

• have a helper that builds a minimal application fixture so I don’t have to recreate the basic structure of an Ember CLI app.
• have a helper that handles symlinking NPM packages into the temp directory so the build has its dependencies.
• be able to pass an absolute path to new EmberApp() so that I don’t have to mess with process.cwd .
• have a public API for resetting internal caches.