node-test-runner: doesn't run on nixos

elm-test appears to rely on native binaries which don’t work on nixos (and potentially other platforms).

$ mkdir tmp
$ cd tmp
$ npm install elm-test
$ ./node_modules/.bin/elm-test init
$ ./node_modules/.bin/elm-test
Success! Compiled 54 modules.                                       
Successfully generated /dev/null
Unhandled exception while running the tests: { Error: spawn /home/avh4/workspace/Apps/tmp/node_modules/elm-test/bin/elm-interface-to-json ENOENT
    at _errnoException (util.js:1024:11)
    at Process.ChildProcess._handle.onexit (internal/child_process.js:190:19)
    at onErrorNT (internal/child_process.js:372:16)
    at _combinedTickCallback (internal/process/next_tick.js:138:11)
    at process._tickCallback (internal/process/next_tick.js:180:9)
  code: 'ENOENT',
  errno: 'ENOENT',
  syscall: 'spawn /home/avh4/workspace/Apps/tmp/node_modules/elm-test/bin/elm-interface-to-json',
  path: '/home/avh4/workspace/Apps/tmp/node_modules/elm-test/bin/elm-interface-to-json',
  spawnargs: [ '--path', '/home/avh4/workspace/Apps/tmp/tests' ] }