Expose override to indicate `ember test` was called.

This is to allow the test command to indicate that it was called.
There is no other simple way to communicate with the `Brocfile.js` (that
I am aware of).

This allows `ember build --environment=production` to not included
tests, but `ember test --environment=production` to include tests.
Add test confirming `ember test --environment=production` works.