Return all unmatched items from the expected output when they are not mached

I based this off of the changes in https://github.com/jirwin/node-transform-stream-test/tree/better-test-result-output

The main change I made is at line 61, where instead of emitting a finished event on a failed case, we'll append the unmatched output to the error message returned:

For test output, see:
https://gist.github.com/phoebesimon/3c4e92a7d7ea8708938e