Changes test script in package.json to use karma logical path

Uses the logical path([npm docs](https://www.npmjs.org/doc/files/npm-folders.html#Executables)) for the executable instead of using the explicit karma bin path.

First suggested by @necolas [here](https://github.com/flightjs/flight/commit/0933abcb4af57ca3c1a65694bf5360f456d7cf55#commitcomment-5829475).