updated double call check on done to provide error to internals.output

When `done` was called twice inside of a test, `undefined` was being printed twice to stderr because internals.output expects an error object and the condition checking for `done` being called twice was providing a string. Thus, the tester has no idea. Ask me how I know... 

I also added an extra carriage return at end of internals.output which keeps output clean when running tests in an automated tool such as [nodemon](https://github.com/remy/nodemon).

Fixes #87