Extend fix for #16 with a scripted test

@tfeng wrote a fix for executing npm correctly for sub modules (https://github.com/sbt/sbt-js-engine/issues/16).

This extends it by a test which can be run via "sbt scripted".

If you revert his patch 705bab8739db7739af5d97fa5ed7c5dc7c474664, it fails.