Add tests/ to npmignore file

[bmuenzenmeyer]

Raphael clued me in on https://docs.npmjs.com/misc/developers#keeping-files-out-of-your-package which we should most definintely be doing for tests/ as normal consumers have no reason to get these files during distribution.

[raphaelokon]

Or add files: ['core/'] to package.json to exclude all explicitly and only include /core into the package :)

[raphaelokon]

Reference here: https://docs.npmjs.com/files/package.json#files