Reject test runners when non-zero exit code

[eliperelman]

No description provided.

[edmorley]

Looks great, thank you!

I wonder if we need some tests for this? If karma/mocha exit zero when no tests found, perhaps this can be as simple as trying to run them with no tests?

[eliperelman]

@edmorley I really want to get to the place where we can have better integration-style tests. Right now all the tests for middleware and presets are sanity-based: as long as neutrino is able to require and use them, we let everything move forward.

It would be nice if preset tests could validate their output: either by running neutrino with the preset and making it doesnt bomb, or validating the actual output.